Is what takes place in the mind, without utterance or the movement of lips, considered an oath, and does it entail expiation?

Waswas (whispers/obsessive thoughts) is an affliction that the one suffering from it must strive against to get rid of it by ignoring it and not paying attention to it, and by asking Allah for well-being. An oath is not عقد (contracted/binding) merely by intention; rather, it must be uttered verbally. In case of doubt whether you uttered an oath or not, the default is the exoneration of your responsibility, meaning you have not sworn an oath.