The problem I see is not that OP is lying, but that he has trouble handling Nel's badgering tone at times, and so he doesn't explain himself as well as he could. I believe this is because, like many disabled people, he's more needy of kindness and more highly emotionally strung than most of us.
He's used to receiving love and sympathy because of his disability and when he's scolded instead, he simply collapses. This is what Nel hasn't grasped or is pretending not to. When he says OP is all about himself, Nel has missed the point. Oscar was always made the centre of attention because of his physical challenges and that was not his fault. But it isn't because he's selfish or self-centered. It's rather because OTHERS have always put his needs first. It's just the life he inherited because of his disability through no fault of his own. But if he only ever thought about himself, he would not have stepped in to save a man's life once.
So, for example, he doesn't explain that when he said he deactivated the alarm it was only because that's what he had concluded when it didn't go off as he walked down the stairs. Not because he was conscious that he did so. He doesn't explain this well because his reasoning faculties are somewhat impaired and yet because in other ways he seems so normal, this point is lost on people.
He is a lovely man, at once and adult and a child, and he is not faking his emotional breakdowns. He grew up more naturally sensitive than most, much like a child with Down's Syndrome.
For the same reason, his responses to situations are not the same as those of a typically reasonable man. Sometimes he does odd things. I've always noticed this in people born with disabilities. Even when those disabilities are purely physical, it often carries over to a certain amount of mental dysfuntion as well. So he doesn't wait for Reeva to reply and does odd things like that.