This is literally how she described him:
Only now, as she looked at him without the pall of danger hanging over her every thought, did she realize how perfectly attractive the man was. His shoulders were broad, his hair soft and shiny-looking, and his gaze bright enough to reveal a keen mind beneath. He was likely a competent thaumaturge. His pristine white shirt cuffs were rolled up to his elbows, and as he crossed his arms she noted the muscles in his forearms and his long, graceful fingers. The vague feeling of attraction made her uncomfortable, and more aware than ever of her transformed body.”
She has never lingered over anyone else like that.
For Stef’s sake (totally not building any ships of my own here, especially after being so adamant previously that the last thing the she needs is a lover ;p) I’ll just keep updating this with the subtle moments Azalea slipped in. Not all of them, since I’ve skipped a couple but…
“But it’s a little more interesting when they’re pretty, don’t you think?”
Familiar dark eyes flashed in Oliver’s mind, but he hummed noncommittally.
The blushy blushy one:
“I thought they were fitting for the queen of ravens, a kind of crown for someone who has no need for gold or jewels,” he added. “Do you want me to help you put them in?”
Sebastien hesitated, but wasn’t sure why, so she handed them back. “Yes, please.”
His fingers were gentle, brushing the rim of her ear as he pushed the wires into her hair.
Her skin burned where he touched, and the wires were cool as they slid against her scalp, weaving into her hair as if alive. She startled.
Oliver chuckled. “It’s an artifact. The wires hold the feathers steady and then conceal themselves, so it looks like the feathers are growing out from your skin.” He stepped away, assessing her, then nodded. “Perfect.”
Her gaze slid away from his. “I’m going to look in the mirror.” She hurried down the hall to the bathroom, where she took a few deep breaths to suppress the frustrating blush in her cheeks. “Don’t be a brainless ninny,” she muttered to herself, scowling at her reflection. She rubbed her ears harshly to rid them of the lingering sensation, then judged the effect of the feathered ornaments.
They did indeed give her a faint air of otherworldliness, even as Sebastien. She could imagine the effect would only be enhanced against the ochre skin and high cheekbones of her face as Siobhan. If only her eyes glowed gold or she had facial tattoos or something similar, the effect would be complete.
After a couple more minutes to make sure she was entirely calm—and there was no way she’d get surprised into blushing again—she returned to Oliver’s study.
Oh and let’s not forget he called her beautiful and set up her fake identity as his lover:
“It was the simplest solution, really. No one will think it strange if Sebastien Siverling occasionally visits a brothel, and Siobhan Naught would fit right in among a group of beautiful, exotic women. It’s the perfect place to hide in plain sight, using people’s unconscious biases and associations against them.” He slipped her a leather-bound booklet. “Identity papers for one Silvia Nakai, declaring you a citizen of Gilbratha. Silvia is legally employed at the Silk Door, and if she gets into any trouble with the law, she can call upon her wealthiest and most influential patron to help her. One Lord Oliver Dryden.” He coughed a little awkwardly.
She thinks he’s endearing:
“I’d like to minimize the death toll as much as possible. All the attacking teams will be supplied with basic emergency aid supplies, but it won’t be enough. Anyone who is seriously injured can retreat or be brought to one of the healing stations to receive more extensive care. Life is precious. Not just ours, but theirs too.”
She held back a small smile. She might not agree with all of his ideas, but there was something endearing about the kind of person who would think like that.
After Newton breaks there’s the whole scene at Oliver’s house where Oliver is sitting next to her bed waiting for her to wake up so he can give her fluids. He’s gentle and caring and gets her to talk about her feelings and fears and thinks about taking her hand but doesn’t (probably because he knows she doesn’t appreciate physical touch).
I never really thought about how sweet a moment that was until I started really paying attention to the relationship between them on this reread.
As the carriage passed by the Verdant Stag, he saw Siobhan. She stood out from the crowd. Although she was wearing a cloak with a hood that disguised most of her physical features, she carried herself with the regality of a queen. Yes, he was sure it was her.
Oliver hummed to himself, feeling ambiguous as he watched her enter the inn-cum-entertainment hall. He had grown closer to her than he planned. He was one to take on “projects,” obviously, and though he’d hoped she would grow to be truly useful—which had happened even sooner than he could have guessed, though not in the way he expected—he hadn’t thought it would be more than that. Yet, now he was worried for her, pleased to see her, and disappointed that he couldn’t stop the carriage on the street and call for her to jump in so that they could talk.
Oliver is slightly more self aware than Siobhan 
That said, he keeps comparing other women to her. A look of defiance from one woman reminds him of Siobhan. He says Tanya’s hair is cut in a similar way to hers but is less striking. He’s really down bad.
Emotionally, Oliver rejected the idea of betraying Siobhan immediately, but he still stopped to consider it. No, he was bound too closely to her to sell her out. She was privy to too many of his vulnerabilities, and neither the University nor the Crowns could be allowed to gain that knowledge. Besides, she was his asset, and he didn’t want to trade her away, no matter what they offered.
My girl! Mine! Hiss!
“I actually prefer it this way. Otherwise it feels like I’m taking advantage of you,” he said solemnly.
She raised an eyebrow. “You were trying to take advantage of me. You would have if I’d let you.”
He grinned. “Exactly.”
Sebastien felt a sudden rush of outrage rising up through her chest, but what burst out instead was a single, breathy laugh that surprised her.
Oliver laughed, too, and gave her a surprisingly warm look. “You’re always interesting.”
When he gave her another present just because:
“I almost forgot! I have something for you. A surprise.” He hurried out of the room with a boyish grin on his face, returning with a ribbon-tied box. “I had them specially made.” He handed Sebastien the box, then sat down across from her, watching her unblinkingly, as if to absorb every movement and reaction.
“Is this for any particular occasion?” Ennis had given her gifts on her birthday, when he didn’t forget, but they were often last-minute, re-gifted items.
“No particular occasion. I thought these would be useful, and the kind of thing you might not think to buy for yourself.”
She tugged at the ribbon, feeling strange. She hoped she wasn’t blushing. “Umm, thank you.”
“You don’t even know what it is yet. Open it.”
As a side note, I really love that he got her a gift that was pretty, comfortable, thoughtful and full of the kind of secrets that she adores. And it was useful for self defence. For all his faults, Oliver really did care about her and clearly spent a lot of time thinking about her.
“What happened? What did they do? Siobhan, talk to me!” Oliver demanded, giving her a little shake as he examined her for damage.
He’s cute when he panics over her safety. I’m kinda getting on this ship with Stef
(but only long term, they both have too much on their plates to be having any kind of romance).
Oliver grimaced. “Maybe. Or maybe one of them just got a little too bold. In any case, you won’t be taking such a risk again.” His hand was still on her elbow, and he took his gaze away from the carriage window to look her over again for damage. “I’m sorry. I never should have agreed to set up a meeting with the Raven Queen.”
She let out a low sigh. “It’s alright. It’s not like we’re any worse off than we were beforehand. If their words can be believed, we now have a censer worth at least a thousand gold. I…overreacted. To the raven getting disintegrated, I mean. It just took me by surprise.”
Oliver was silent for a moment, and then his hand slid down from her elbow, gripping her smaller hand in his and squeezing.
Dear god, is this unprotected hand holding?!
(Well, they might be wearing gloves, but a girl can dream.)
She let the conversation die, taking some comfort in the anchor of Oliver’s hand against hers.
Sigh
She closed it and locked it behind him, then turned to find his eyes trailing slowly over her form. Sebastien’s clothes were too tight in some places and too loose in others, and she had rolled up the hems of the pants and sleeves so they didn’t drag.
Oliver raised his gaze to meet hers without a hint of apology, giving her a questioning look.
“I’m doing some tests on the transformation,” she explained. “I thought I might as well make myself useful while I’m at it.”
“You’re lucky I didn’t immediately assume you were a burglar and kick down the door. If I hadn’t smelled your brewing from the hallway, I might have.”
She rubbed the back of her head awkwardly, fingers tangling in her long hair and forcing her to tug them free with a wince. “Oops? I hope you don’t mind that I spent the night again?”
His expression flickered too fast for her to track, some unknown emotion or automatic response quickly suppressed. He hesitated slightly too long, just staring at her, then said, “Not at all.”
She raised one eyebrow. He gave her a slightly lopsided smile. “You should know you’re welcome to spend the night any time you like.”
Siobhan blinked once, and then the possible double entendre filled her with horrid, belated embarrassment. She hurried around him and back to the alchemy station to continue her work, thankful that her darker skin would not show a blush like Sebastien’s. It took a few moments for her to regain full focus on the magic, but she managed, and the next time she looked away from the soup pot, Oliver was at his desk writing a letter, seeming to have forgotten her presence entirely.
Never let it be said that our boy didn’t drop any hints.
Oliver chuckled at her sparkly-eyed look of anticipation. “I hope you’ll still stop by to brew for me every now and again, even if you don’t need to?”
She turned back to Sebastien as Oliver watched. “Well, it is good practice, and I’ll need to restock and expand my own potion supply anyway.”
He smiled ruefully as she left, shaking his head.
That’s practically a “come visit?”
Okay, this next one made me cackle:
Oliver was there when she arrived, but seeing that she was busy and distracted, he said only, “Have breakfast with me in the morning before you head back. There’s some upcoming work I want to talk to you about.”
He has this hobo wandering into his house, realises she’s too invested in her latest project to talk to him but is perfectly happy to use his house as her own and he’s not only fine with it, he’s inviting her to breakfast 
During the battle at Knave Knoll she’s terrified for Oliver and her concentration keeps wavering as she tries to see what’s happening to him.
He’s also worried about her:
They caught up with the main group after a few minutes, and even though she couldn’t see Oliver’s face through his mask, she watched as some of the worry in his shoulders dropped away when he caught sight of her.
The painful part:
She wanted to ask Oliver what he had that the University hadn’t found. She wanted to confirm her suspicions. She also wanted to flip over his desk, pin him to the wall with her forearm, and scream for him to tell her the truth.
In another way, however, the enormity of this suspicion, this revelation, went beyond any pain her tongue could inflict in return. Words felt too inadequate a response, and that helped her suppress them entirely.
Honestly, if you compare and contrast with the other betrayals she suffered, you can see she’s so much angrier at Oliver because she really does like him. When Lacer betrays her or lets her down she’s disappointed. When Ennis, the man she loved, lets her down for the last and worst time she screams herself hoarse. Her reaction to Oliver betraying her is much closer to how she reacts to her father because her feelings for him are more intense.
Oliver was partly responsible for her ruining her life but that was before he knew her and wasn’t targeted at her.
Lacer tried to violate her mind and she wasn’t so blindingly angry at him. Her heart hurts with Oliver 
“Has Oliver Dryden ever made you uncomfortable, flirted with you, or attempted to get you to repay his help with sexual favors? Is he trying to coerce you into some sort of relationship?”
“What? No. What?” Sebastien had spoken before even considering her response, reeling from the completely unexpected line of questioning.
Damien stared at her searchingly. “Are you sure?”
For a moment, Sebastien considered the gifts Oliver had given her, the meals they had eaten together, and the conversations they’d had. But Oliver treated everyone like they were important and special.
So she does feel important and special when Oliver talks to her 
And if he had also thought to take some vindictive pleasure in seeing Siobhan’s father get what he deserved, surely no one would judge Oliver for that? He had been at ease, because Siobhan knew of the danger the day presented and would stay safe under Liza’s wards.
The worst Oliver had imagined happening was that Ennis Naught might be sentenced to death, and Siobhan, despite her disdain and resentment for the man, would be distraught at the fate of her father.
He cares.
Something grew sick inside of Oliver when the raven delivered its letter to the center of the Edictum Council floor. Surely…someone was framing the Raven Queen? Taking advantage of her reputation, just as Oliver had speculated might be possible. Either that, or things had gone desperately wrong. Was Siobhan turning herself in? Wild ideas spiraled through Oliver’s head like debris carried within a tornado.
If Oliver’s dread were tangible, it would have been slicing into his internal organs with every breath. They were at the top of a precipice now, and he could see no way of stopping their descent. Not when she looked like that.
God, I can feel this.
Sebastien’s face was emotionless, and for the first time her eyes reminded him of those of a shark: cold-blooded, predatory, and uncaring. Responses ran through Oliver’s mind, different ways to try and mitigate disaster, to hold up the crumbling brick of their relationship, built so gradually and now tearing apart.

“Listen. I will tell you once.” He paused, drew a deep breath, and repeated more softly. “Please listen. I didn’t intend things to work out the way they did. I never planned for Siobhan Naught to steal a book in my stead, or become the Raven Queen, or my friend. I may not have shared all of my secrets with you, but that has never been a requirement of our relationship, never a promise I made.”
The echo of the front door, though it had been closed gently, seemed to reverberate through Oliver’s bones. He forced himself not to watch her leave through the window. Instead, he pressed his trembling fingers to the cool wood of his desk. Then he let his head slump down onto his hands.
He had lost something precious, and it was more bitter than he had ever imagined.
Honestly, this part always takes my breath away. It’s quite gut wrenching to read when I have so much sympathy for them both. He’s been sick with anxiety, knowing something has gone wrong, he’s angry that she’s cutting him out, he’s worried about losing what they have - that precious, carefully built relationship - and knowing that he is being honest but that she’s too paranoid to listen and that she’s angrier because she trusted him and less inclined to listen and believe him because of that.
She’s assumed that guilt of one part makes for guilt of the whole and she has a whole lifetime of experience being betrayed by the people she loves. She sees the pattern repeating and just wants to hate him wholeheartedly so he can’t hurt her like everyone else did.
His comment about the lopsidedness of their relationship flashed through her mind, and some miserable emotion that was too complex to identify wriggled through her chest. Maybe that part was true, a little.
I hate how they both suffer, even though it gives me hope that in the future they will actually have learned from this and get together in a much healthier relationship with more open and honest communication.
“Of course. Are you going to stay the night? We haven’t touched your room.”
Sebastien shook her head. “I have other accommodations.”
She shook her student token impatiently, and Oliver moved forward to get it. He stared into her eyes as he took it, his fingers brushing against hers. “Thank you.”
I thought this was so cute, she’s still so mad at him but she really went out of her way to help him when she thought it was something that would impact him and he was there like “I kept your room for you
” and then he went on to ask if she was feeling ok after her father escaped from jail and then:
Oliver very obviously swallowed back whatever words he wanted to say. “I have to go.” He hesitated, then added, “You are welcome to stay here, if you wish. Whenever.”
This is him just outright going “I miss you, please come home” 
Sebastien didn’t look back. After about a block, her pounding heart settled. She paused after turning the corner and rubbed at her shoulders, neck, and the sides of her jaw to release the tension there. “Damn you, Oliver,” she muttered. What right did he have to act like he cared?
Oh Siobhan 
He looked down at her, his dark blue eyes shadowed and a lock of hair falling forward across his forehead. He remained silent for a few long seconds, his gaze trailing across her face gently before returning to meet her own. “Have you already considered my offer, then?” he asked, his voice as soft as his nameless blend of an accent, and something inscrutable in his expression.
She’s seriously aware of him.
Siobhan let out a short, sharp laugh. “That’s what Thaddeus said! Professor Lacer, I mean,” she corrected.
I’ve been wondering since the first time I read that if she was trying, just subconsciously, to make him a little jealous with that line.
Sebastien should be with someone lighthearted, kind, and outgoing. As part of a couple, Sebastien should be the mature, serious, driven one. Otherwise, his relationship would probably consist of nothing more than studying, discussing, and practicing magic, with nary a romantic moment. There was a good reason for the saying, “opposites attract,” in Damien’s opinion.
So, if you combine this with Sebastien having said that she’d want someone older who you could have a sensible discussion with:
Oliver is older, he’s also charming, he makes her laugh and she has spent hours talking with him and lingering over food to enjoy his company. He brings her out of her shell and there have been times he’s enticed her out of her room when she was going to lock herself up and study. She respects him, but she doesn’t view him as a teacher or father figure. She does find him sexy - so far he’s the only one she’s mentioned an attraction to in the whole series - and despite them both being driven and having their own goals they’ve had romantic moments from time to time despite both of them being incredibly careful not to reveal their feelings, like a pair of wary cats circling each other.