Weird first post but hear me out:
Bonus questions: what happened to his sister? She was a Null so is he also a Null?
Super weird but I’m 100% here for it.
Oliver, secretly a fox shapeshifter? It would explain his previous experience and comfort with Lynwood!
No.
for some reason there is a 20 character limit.
I think he’s null, or null adjacent. If he was a shifter, I’d have thought “stag” would be the animal that would make more sense - but the clues from the big fight on the road (potions don’t work well on him) lead me to null. He also has a large library on magic. It would seem ridiculous that he wouldn’t have sufficient will to be able to cast otherwise. And, the University accepts all ages over the minimum; it feels like he could have attended if not just bought lessons.
I mean, is it possible that nulls are actually just bloodlines that have interbred with other species, but retained the magic of neither?
That would actually explain quite a bit in some ways, if sometimes when you mix species you get the magic of both, and sometimes they sort of cancel out, or a child just doesn’t get enough of either’s magical traits, and they end up not being able to use either style?
In that case Oliver might be part kitsune, and Siobhan might conceivably be part brillig, with the People having interbred with them secretly in the distant past and the Blood Emporer having launched a program to try and concentrate these bloodlines so their hybrid traits re-emerged?
Various foxlike Oliver references:
He grinned like a fox, the edges of his lips curling up a little too far in a way that made her think of skinjackers and the cautionary tales mothers recited for children before bed. “You can call me Mr. Dryden. Let me take you to my associates. We can speak more there, out of the dark and the damp.”
His mouth twisted into that vulpine smile again. “You misunderstand. I’m helping myself. Any benefit to you is incidental.”
Underneath his mask, Oliver’s face broke into a wide, foxlike grin.
The edges of Oliver’s mouth curled up a little too far in a way he knew made him look vulpine, but Lynwood didn’t seem disturbed, his own lips pulling back to reveal sharp-edged teeth.
Oliver stepped back, letting his huge, foxlike smile recede in favor of a grimace of distaste. “You taint everything you touch,” he murmured. “What a waste.”
We also know that kitsune are a thing in this world as a presumably sapient species
Sebastien caught a glimpse of a coy kitsune slipping through the crowd with her fluffy tails wrapped around her body.
And there is even some indication that kitsune have interbred with humans, the case of Agent Marcurio, described as a man with two fluffy tails.
On the right, their boots still muddy, waited two men, one with ginger-orange hair and another with a pair of furry tails. All four wore the red shield symbol of their order and held a clipboard, the paperwork already filled out and reviewed.
And
Marcurio also focused on her with avid interest, his fluffy tails swishing back and forth eagerly.
Kitsune in folklore are magical, intelligent fox spirits that have as many as nine tails and can at least assume the form of a humanoid, and it seems like that in the PGTS universe they’re multi-tailed fox people or shifters with their own form of magic.
Basically, yeah, I agree now that Oliver is part fox and that might be precisely why being a null runs in his family.
This is an S-tier theory. Thanks for gathering evidence!
Did. . . did you have those ready and highlighted, just for this? Inquiring minds must know.
Lol, I was doing a re-read, noticed one of these, and decided to do an ebook search to find the rest.
You’re the best. 
Well now I want him to be, whether it is canon or not.
I’m not against the idea he’s a null or some sort of null-adjacent. But maybe the experiments don’t to him were not great/ don’t have a benefit. Potions not working well on him could be because in order to fight off whatever ails him he must take many potions and as such they lose their effectiveness?
I would support him being a fox shifter.