Is it permissible for someone who has recovered from obsessive-compulsive disorder to continue adopting the lenient rulings (rukhsas) that were permissible for him during his period of suffering from the disorder, and would his acts of worship be invalidated if he were to continue acting upon these rulings?

It is permissible to seek a concession (rukhsa) out of need, and this includes obsessive doubts (waswasah). As for someone who is not afflicted by obsessive doubts, they should not intentionally seek out concessions; rather, they should follow a scholar whom they trust. If someone believes something to be obligatory and then seeks a concession by following the opinion of someone who does not consider it obligatory, while still holding their original belief in its obligatoriness, their prayer becomes invalid.