a)Rand touched the OP without knowing it several times in the beginning. Granted, that was when he did not yet know he was a channeler, but some form of madness could easily make an insane male forget and use OP without really knowing which would bypass the oath.

b)It would not actually do any good and would be no different from the easier approach of being gentled. OP is so addictive that virtually everyone who have experienced it must have it again and again or they lose all hope and die. Making an oath to never touch OP would eventually kill the man from lack of OP, just as surely as being gentled would.