The correct form would be: I'm the only one who hasn't passed yet. However, I would like to mention that in formal writing you do not use contractions (combinations of two words, utilizing an apostrophe). Also, if this is intended as a question, I think it would be better phrased as follows: Am I the only one who has not passed (insert whatever you are asking about) yet? I hope this helps.

hasn't who = 3rd person singular