Aloha Krisjon,
Welcome to Oahu! As you are finding out, there are far fewer accessible off-roading possibilities on Oahu as compared to the other Hawaiian islands. However, if off-roading is your passion, I suggest getting in touch with Jamie through:
http://hawaiianhardbody.com He and his friends in the Hawaiian Trail Riders Club will be able introduce you whatever off-roading is available on Oahu.
http://p209.ezboard.com/bhawaiianhardbody