Oahu is a great island which offers lots to do and see. It is the busiest of the Hawaiian Islands with the Cities of Honolulu and Waikiki being the main tourist attraction. Oahu is a great location for bigger groups or if you like shopping and having lots of amenities close by.
You can certainly get away from the city. Oahu's famous North Shore offers some fantastic beaches and during the winter months hosts professional surfing at Sunset Beach and Pipeline.