That’s actually the correct way to write O’ahu. When writing Hawaiian language words, the apostrophe is called an okina, and indicates a guttural stop when speaking. So the proper pronunciation of a word like O’ahu is aided by the presence of the okina (apostrophe.)
As a former Hawai’i resident (yes, there’s supposed to be a guttural stop in that word, too) I do appreciate AvWeb respecting the language and the local culture and trying to get it right, despite the snarky comments to the contrary.