If you haven't booked your airfare and/or hotel yet, you may consider looking at the online travel agencies (Expedia, Orbitz, Travelocity, etc.) as sometimes they offer good deals when you combine car rental with the other stuff at the same time. This is what I did with Maui.
Also, if you will be staying in Waikiki (where most of Oahu's hotel rooms are located) you may want to reconsider having a car. Most if not all the hotels charge extra for parking and some don't allow repeated in/out (meaning you pay each time you park). Street parking is near-impossible to find and traffic is awful especially on Friday and Saturday evenings. If you need a car to visit other parts of the island you may want to limit it to the few days you really need one and plan the trip accordingly.