I honestly don't know how I write Oliver ngl, I just write him according to how I think he acts in bluelock + I based him off on the MLs of different college aus I've read (I think the last one is what actually helped out a lot lmaoo)
But before writing the actual story I do try to make notes on how his character (personality, where he lives, who his friends are, etc.) will be so I have something I'm constantly looking back on.