We stayed in Oia for 4 days (really short for Oia, but didn't have much time) and we had 2 meals at Roka and both meals were perfect! If we stayed longer in Oia, I guess we would have gone there even more. It was a bit difficult to find at the beginning, but if you use your Google map, you will get there. it's quite near from the Oia bus station.
Oia is not cheap place to stay. Most restaurants charge high price for food and drinks.
However, Roka provide good food at very reasonable price. Must visit restaurant for Oia!