Job Fairs will attract different students than OCIs. For example, I go to UT. The top 10% (maybe more) of the class should have very little problem getting some top job they want through OCI. Those people have little reason to do job fairs unless it is targetting some specific practice or a specific location so they might not otherwise meet those employers (in our case, UT people might want to see a bunch of Chicago or New York firms, and think not enough come to OCI). Generally, though, you'll probably not be competing against the very top students at a job fair.
Job Fairs will also have people from different schools. So if you are at the only top 50 school within 4 hours, but there are 4 schools in the 100's nearby, you will probably have better success rates at the job fair compared to OCI.