I wear an 8 and an 8 is really nearing too small on the heal area. My heels dang near hang off, but the 9 was too big and the straps were too lose. If you're a 8.5 go with a 9. Even people who wear an 8 might want to go with a 9. But I don't like having too much room on the back of my flip flops. Plus, the bigger size felt hard to hang onto my foot and my toes were grasping to hold them on as I walked. I got stuck with an 8 and its working out ok. The more i wear them the more the give and more space is made available on the back.
I just bought a pair at the store and tried on several styles of OluKai. I am a pretty true 8.5 in regular shoes but flip flops are a little different and subject to how you like yours to fit. I found that in the Ohana due to the wider straps, I am an 8 (so down a half size) but in the other styles with thinner straps I need a size 9 (so up a half size).