Bus vs rental car on Oahu (honolulu)

It depends on how long your stay is. If it's just a few days and you want to see a lo, you should rent a car, it's like $50-60 a day, but a moped is more advisable, it's $35 a day, with discounted prices for several days rent, and you won't have parking problems. Buses are a good option too, there are many of them running every day, a fare is $2 and they cover most parts of the island. The central hub is near the Ala Moanna Center, you'll find all information there.