A recent study finds that only 20% of a convenience sample of teens consider oral sex to be sex. This finding has been documented anecdotally, such as in the classic scene from the 1994 movie _Clerks_, where the main character is described to hear the discrepancy in his girlfriend's number of sexual partners and number of oral sex partners. (3 vs. 37.) He had only been told about the former.
Of course there's the caveat that this convenience sample has no implications about US teens' norms in general since the sample wasn't random.