We had a great time in Oia at this cave house. If you're looking for something simple, authentic and in the heart of Oia, this is a good option. Definitely not luxurious, but comfortable and clean in a simple, no-frills way. View is beautiful too, although you won't see exactly the sunset perfectly (i.e. sun disappearing into the sea) directly from the balcony - it's visible just a few steps away. Having looked at several other options, this seems like a good value-for-money compromise to me!
Last but not least, Maria and her staff were extremely helpful.