It’s very easy to get around in Santorini. There is an efficient and quite simple public solution. So, the facts are:
– there is no direct interconnection between the “small” cities (Oia, Kamari, etc), all those cities have interconnection only with the main city named Fira. So, this is like a “star” system. If you live in one of those smaller settlements, you will always travel to Fira, and the from Fira to another settlement of Your goal.
– the last station to Fira is the main bus station. You can take the bus lines to all the other settelements from there.
Here is a map of Santorini (by the way locally also called Thira) bus lines:
– bus tickets (at the moment) costs between 1.5 and 1.8 Euros for one destination.
– you can buy ticket only on the bus
– the Greeeks are extremely hospitable, and during my time there I newer saw any K-TEL (K-tel is by the way the Greek official public bus company) officer on the bus to refuse to help or give information. They do speak english. They do tell/shout when to get off the bus if you ask them for Your destination and explain them that you’re not sure where to get off.
And that’s it. As you can see on the map, you can reach all of the main island via public buses and it’s very cheap! You don’t really need a rentacar at this place.
As for the frequency of the buses is considered, I guess that is due to change, depending on season. Here’s a photohraph I made about the bus schedules in September 2014: