Not much in the way of dirt roads outside of the off-limits pineapple plantations. Kaena pt and Peacock Flats Jeep trails are open to you once you are legally registered in the DLNR system. Kahuku Motocross/ATV trails are good from what I hear but I ride a big pig that isn't meant for going straight up the side of those hills. Oahu is still more than 60% un-developed. Won't find anything on the internet or in the forums about non-DLNR areas but they exist - unless you are on a KTM 640A or Exc 500 most of them are 350 or smaller trails/roads on full knobbies only and you got to sit in slab-side commuter traffic jams to get to trail entrances. For the kind of riding you (and I for that matter) would like, Big Island is best - not much 'open' dirt left on Oahu.