If Billy had OCD, I think it would manifest like this:
(tw - discusses violent intrusive thoughts and self harm)
A lot of his compulsions would be invisible, or subtle. They were more obvious as a child, but he learned to hide them because Neil would get mad at him for showing them. Most of the time he’s ruminating. When he’s deep into ruminating, people tell him he looks pissed off. However, he’s actually stressed. He doesn’t realize how it looks from the outside, and he doesn’t intend to look angry.
Billy blocks out intrusive thoughts with loud music. He will play the same songs over and over until it’s just right. When Neil is home, he listens with his headphones instead. He will rewind the tape to the same spot and replay it multiple times when the intrusive thoughts are really bad. The whole process is upsetting, and occasionally he cries from the distress.
A lot of Billy’s intrusive thoughts are extremely violent and disturbing. Sometimes they involve hurting the few people he cares about, or even himself. Intentionally crashing his car. Sometimes his mother is inside. He knows he can’t hurt her, because she’s not there. However, sometimes Max is inside. On days like that, he avoids her as much as possible. Protests when his dad makes him drive her anywhere.
Then he stresses over whether or not he will become like his father. Anytime he loses control of his temper, he isolates himself and replays what happened over and over in his head “checking” for anything resembling Neil. He will pace. Bite his nail beds. Pick at his skin. Smoke a pack. Checking. Over and over and over and over and over and over again.
Billy has a lot of checking compulsions. He checks locks multiple times. He will check his bedroom door multiple times a night to see if it will still open. The few times it doesn’t because Neil’s locked it from the outside, Billy panics. Hides himself in the corner. Being alone with his thoughts is the worst place to be. He’ll hold his mother’s necklace and prays for them to go away. Rocks himself to sleep.
Billy hoards things, too. Random piles of junk everywhere. It doesn’t help that Neil makes him periodically get rid of things. Billy internalizes the idea that if he’s not careful enough then his things will disappear. Billy gets attached to things easily. Much more easily than he gets attached to people. When he loses a sentimental item, it’s like a loved one has died.
Billy is particular about time. Part of it is because he’s got to be, because Neil gets mad when he’s not on time. Billy rarely loses track of time, though, unless he’s wasted. He plans out everything in his brain down to the second. Says a lot of ‘If by x, then y’ statements to Max and to himself. He hates how Neil doesn’t respect his time. He can be three hours late, but Billy’s punished if he stays out too long.
The Mindflayer is like his OCD manifested. It invades his mind, telling him to say and do things he thinks are repulsive. It’s his worst fear come to life. When it’s all over, Billy gains a new compulsion - checking over his body for remnants of it. Convinces himself it’s swimming in his veins. When it’s really bad, he thinks about taking a sharp object to his veins to cut it out.
The good thing is this time around Billy gets to go to therapy, mostly because of what happened to him but his OCD comes to light in the process. After suffering silently for years, he gets an official diagnosis. His newfound friends and family accommodate him, and he knows he doesn’t have to isolate himself when the intrusive thoughts get bad.
They remind him he fought The Mindflayer and won, so he can win this battle too.