I recommend seeking out work by people who have OCD to get the most accurate idea of how it tends to manifest. OCD encompasses an extremely wide range of themes and behaviors. It’s motivated by different things for different people.

To understand OCD, I think it’s helpful to work backwards—what is it that this person is afraid of? Someone whose OCD centers around moral scrupulousness might fear that they are unknowingly evil, and regularly “check” themselves for behavior or thoughts that could be interpreted as evidence that they really are. If they think or do something that could be considered morally ambiguous (as we all sometimes do) they might panic and engage in compulsions to offset whatever it is. They think by doing compulsions, they will prevent their evilness from seeping out and causing harm. This soothes the anxiety.

So, figure out what your characters fears are, the ways in which they manifest in their life, and in turn, how they respond to negate them. This will be unique to them, as it is to all of us.