Is cutting one's hair considered disobedience to God when there is doubt about an oath after performing Istikhara prayer and seeing supportive dreams regarding the cut?

You must abandon obsessive thoughts and pay no attention to them. As long as you doubt that you swore an oath concerning the mentioned matter, the default is that you did not swear an oath, and there is nothing upon you. You must not succumb to obsessive thoughts, as they lead to great difficulty.