Oia is the choice of the romantics , very picturesque and artistic, quieter than Fira, but easy enough to hop a bus or grab a taxi into town to visit the capital. Unless you're visiting Santorini to party until dawn, Oia is the place to stay, and has been designated a traditional settlement. Oia is on the northern tip of the island, built on a steep slope of the caldera.
Oia is "famous" for it's sunsets (although really, that's just marketing hype because they are equally as spectacular many places in the Greek Islands ), and it gets quite busy around sundown as visitors pour in from all over the island. There is quite a respectful hush over the town at the edge of the island when the sun sets into the Aegean.
Oia suffered badly from the 1956 earthquake, and the remains are quite evident, adding to the feeling that you have wandered into Escher's impossible staircase painting. Many of the buildings are built into niches in the volcanic rock (they are known as hyposkafa), and it is easy to get yourself lost and end up on the roof of the place you're trying to find. It's quite a delightful experience though, and you never know what you'll stumble upon so relax and enjoy yourself!
The streets are charming narrow passageways that twist their way along the rim of the caldera, and down the slope of the cliff. There are plenty of tourist shops, but they are of a different, more artisan, quality than the more traditional tourist offerings of Fira, and there rare also a number of beautiful commercial galleries. I still fantasize about many of the things I saw in Oia, most especially the exquisite chunky amber jewelry.