Okay to be completely honest, why Oahu? We did three islands on our trip, Maui, Hawai'i, and Oahu. We found Oahu to be the worst, it was the most commercialized, the beaches were horrible (Waikiki had a surf breaker in, there were no waves it was just like a saltwater swimming pool and it was ridiculously crowded) and everything was more expensive.
I agree with this person if you only stay in Waikiki! The south shore is not known for it's waves like the North Shore is.
Overall, Oahu has so much to offer. The North Shore has some of the most famous surf beaches- Sunset Beach, Banzai Pipeline and Waimea. Haliewa is a quaint little town with some great hole in the wall restaurants and the famed Masumoto's Shave Ice! You can hike a paved trail while pushing a baby stroller near Makapu'u lighthouse just east of Waikiki and get a great view of the east side of the island and possibly Maui in the distance. Then you can drive up the Windward side north and see rocky beaches/cliffs and head up to the Polynesian Culture center for one of the best night shows on the island. Diamond Head Crater is also convenient to Waikiki a great hike to see all off Waikiki and Hanauma Bay is also nearby and has some awesome snorkeling. Then there is the history on the island between Pearl Harbor, the USS Missouri and Punch Bowl Cemetery AKA National Memorial Cemetery of the Pacific.
Maui has the incredible Road to Hana with the different colored sand beaches and Haleakela crater for sunrise, Kauai has an incredible north shore mainly viewed from a boat and Waimea Canyon, along with some great beaches and Bubba's Burgers. The Big Island has Hawaii Volcanoes National Park- perfect for seeing the lava flowing into the ocean and a ride around the southern portion of the island will get you to the southern most point in any US state as you head from Hilo towards the seaside town of Kona (famous for the coffee).
You really can't go wrong with any island! Enjoy your stay!
Aloha!