What is the ruling on one who doubts having made up a missed prayer due to obsessive-compulsive disorder (OCD) that does not always afflict him, coupled with the strong presumption that he has made it up?

Whoever doubts having performed the prayer, the basic principle is that he has not performed it, and he is obligated to make it up. However, if he is afflicted with obsessive whispers (waswas), he is not required to make it up if he predominantly believes he prayed, then doubts. Rather, he should ignore those whispers.