We live in California, so gas, grocery shopping, and dining on Oahu is not a huge sticker shock. If you live in middle America, it may be quite a shock.
There is a little strip mall behind the resort that is pretty reasonable for food, but I think it will become tiresome after a few days.
Eating on site is terribly expensive ($18 for a burger).
A car at Ko Olina is highly recommended. Best provisions are at Walmart (a few miles away) and Safeway. Costco is also near those.
In the area of the above shopping are tons of eating establishments you will recognize. Some places like KFC are outrageously expensive, while McDonalds won't be.
If you do rent a car, make sure to leave extra time to drop it off and take the shuttle to the airport. The drive from Ko Olina to the car rental is 30-40 min with NO traffic. Plan for traffic. Plan to get lost. Plan for a 20 min shuttle. Don't miss your flight because of returning a rental car. Also, you may want to consider pre-paying for a full tank, as it can be difficult to find a gas station near the rental agency.
The excursion I would most recommend is Captain Bob's Sandbar Snorkel Adventure. It's on the opposite side of the island but is a fantastic drive. Visit the Pali Lookout on the way back.
Also recommended is Kualoa Ranch, which is also in that area.