While I think Ohio State and USC have stronger resumes than Alabama, I think the committee will favor 1-loss 'Bama over the 2-loss Buckeyes and Trojans. There's precedent for the CFP committee taking a team that didn't qualify for its conference-championship game, as Ohio State made it over Penn State last season.
I also think the committee — which strives to put together the best possible matchups in the playoff — is mindful of the fact that Clemson shutout Ohio State in a non-competitive semifinal game last year while Clemson-Alabama in the final was one of the most memorable championship games in college football history.