What is the ruling on someone who swore by saying, "By our Lord, I will not repeat it," due to obsessive-compulsive disorder (OCD) in prayer, then doubted the validity of his prayer and decided to repeat it without expiation?

Your oath is not a yamin ghamus (a perjurious oath that plunges one into sin). If you swore while overwhelmed by waswas (obsessive whisperings), then no expiation is binding upon you, because you are in the position of one coerced. However, if you swore by your own choice, without the influence of waswas, then kaffarat yamin (oath expiation) is binding upon you for breaking your oath, as you swore not to repeat the prayer but then repeated it. Ibn Qudamah said: "Whoever swears to do something but does not do it, or not to do something but does it, must offer expiation. There is no disagreement on this among the jurists of the various regions."