Is a person who swears by Allah involuntarily due to obsessive-compulsive disorder, and then fails to fulfill what they swore upon, punishable? And what is the best solution in this case?

Praise be to Allah for your recovery from the illness that disrupted your worship. As for the internal monologue of swearing, it has no effect and entails nothing. One should disregard such thoughts lest they develop into a psychological illness.