Is an expiation (kaffarah) for a broken oath obligatory for someone who swore by Allah that if he were to interrupt his prayer, he would be an infidel or would have worshipped other than Allah, and then he interrupted it? And is expiation obligatory when there is doubt about the wording of the oath? And is it permissible to delay it?

It is clear from the question that swearing to disbelief by abandoning prayer to ward off obsessive thoughts is a sin requiring repentance, and the swearer is not considered an unbeliever merely by this statement.

There is a disagreement among scholars regarding the necessity of expiation. If you doubt whether the oath was uttered, the default is the acquittal of responsibility.

If you wish to offer expiation but cannot find needy persons, appoint someone to give it on your behalf. If you are unable to do so, then fast for three days.

One must beware of obsessive thoughts, strive against them, and not yield to them.