What are the rulings concerning repeated oaths requiring expiation, and is it permissible to fast instead of feeding the poor, and what about oaths made in the heart or resulting from obsessive-compulsive disorder, and is expiation for an oath obligatory when repeating prayer due to obsessive thoughts, despite having sworn not to repeat it?

A Muslim should not make frequent oaths. Swearing an oath unintentionally is considered a 'laghw al-yameen' (a thoughtless or vain oath) and does not require expiation. Similarly, an oath made in one's heart does not require expiation, because an oath is only binding if articulated verbally. If you doubt whether you verbally made an oath or not, no expiation is binding upon you. As for swearing an oath while angry: if you intended what you said, then the oath is binding. Your oath not to repeat the prayer, followed by repeating it, does not require expiation as long as it was due to the influence of waswasah (whispers of Satan or obsessive thoughts). Fasting is not sufficient for expiation of an oath if one is able to feed the poor. If you have enough to feed the poor, then that is what is obligatory upon you. If you have enough to expiate some of the oaths, then feed for those oaths and fast for the rest. However, if you do not have enough for any of that, then fasting is permissible. You should hasten to pay the expiation. If you cannot expiate except by fasting, then fast as much as you can starting now, and complete the rest after Ramadan.