The fastest way to get to Osan Air Base from Camp Casey is the train. You will take line 1 at Bosan Station, right outside the gate, and go south. Do not transfer lines. Get off at Songtan Station, once you get there ask a cab driver to take you to the main gate. It will be about 3,000 won for the cab ride, another 3,000 for the train, and about three and a half hours of travel, the trip is worth it though, Osan's "Ville" is huge (Yes, it's way bigger than Casey's) so plan ahead and take some money with you for the souvenirs and if you plan to spend the night (highly recommended). Hope this helps.