I got a handy little guide from Hawaii Audubon years ago (by mail) that includes both endemic and introduced species, with suggestions for where to go on all of the islands. It's a small booklet, so good for travel. I haven't been to Oahu, but I can recommend the Waikamoi Preserve on Maui (you need a reservation to go in with a guide) and the Alakai Swamp trail on Kauai for endemics, if you're able to get to any other islands. Enjoy!
Patricia Smith
Seattle & Wenatchee, WA