Oia is about 15 minutes away from Fira by cab. I agree with Karen, you really do not need a guide for this island, and that's a lot of money! I loved Oia. It's where you see more of the white-washed blue-domed buildings, and the views over the sea are even better than in Fira. It is also much less commercial. Oia and Akrotiri are on extreme opposite ends of the island and both worth seeing. If the guys don't mind driving, why not rent a car? You are there in off season, so the traffic will not be heavy at all.