Our best advice before going to Oahu is to get the Oahu Revealed book. You can get it here. That group of Hawaii travel guides is hands down the best guide books we’ve seen for any destination. You’ll see other people with the book all over the island.
If you enjoy snorkeling, we highly recommend buying your own snorkeling gear, you can do so here. It will be cheaper than renting gear and in a place like Oahu, there are dozens of great snorkeling spots you can do free if you have your own gear. You just go to the beach, put on your gear, and hop in the water.
Oahu is fairly small as far as the land it covers, but it is by far the most poulated Hawaiian Island because it is home to Hawaii's biggest city - Honolulu. Most people who come to Hawaii end up staying in Honolulu, which is home to some famous landmarks such as Waikiki Beach and Pearl Harbor, and it is very close to places like Diamond Head, a volanic cone you can hike up.
The lesser known part of the island is the North Shore, where the tall buildings of Honolulu are left behind for lush vegetation and big waves for surfing.
Something else to be aware of about Oahu is one common complaint is traffic, especially in Honolulu. There are a lot of cars and a lot of people in a small, landlocked area, so just be prepared. If you are looking for a place with a lot less traffic and fewer crowds, then the Big Island may be the place for you. Then again, on the Big Island you may end up driving for hours just to get from one end of the island to the other.