Oia or Ia is a small village and former community in the South Aegean on the islands of Thira (Santorini) and Therasia, in the Cyclades, Greece. Since the 2011 local government reform it has been part of the municipality of Santorini, of which it is a municipal unit. It covers the whole island of Therasia and the northwesternmost part of Santorini, which it shares with the municipal unit of Santorini. The main street is named Nikolaou Nomikou. The population was 1,087 inhabitants at the 2021 census, and the land area is 19.449 km2.

Santorini, officially Thira and Classical Greek Thera, is an island in the southern Aegean Sea, about 200 km (120 mi) southeast from the Greek mainland. It is the largest island of a small circular archipelago, which bears the same name and is the remnant of a caldera. It is the southernmost member of the Cyclades group of islands, with an area of approximately 73 km2 (28 sq mi) and a 2021 census population of 15,480. The municipality of Santorini includes the inhabited islands of Santorini and Therasia, as well as the uninhabited islands of Nea Kameni, Palaia Kameni, Aspronisi and Christiana. The total land area is 90.623 km2 (34.990 sq mi). Santorini is part of the Thira regional unit.