Have I committed major shirk by saying "O Sheikh so-and-so," even though I did not intend to seek aid or intercession, but rather said it out of forgetfulness, courtesy, or confusion? And have my deeds been nullified because of that?

There is nothing against you in calling out to that particular Sheikh, as long as you corrected yourself and called upon Allah, the Most Glorified, and did not intend to seek aid from the Sheikh, especially since you suffer from obsessive-compulsive disorder. It is forbidden to go to a raqi (exorcist) who uses polytheistic practices in his treatment, and one must beware of him and warn people against his shirk (definition) (polytheism).