The municipality Oropos stretches between the Parnitha mountains and the South Euboean Gulf, opposite Eretria (on the island Euboea). The town Oropos, the seat of the municipality, is situated on the lower course of the river Asopos, 4 km south of the coast. It lies 4 km southwest of Nea Palatia and 36 km north of Athens. The community Oropos consists of the town Oropos and the nearby villages Kampos and Platania.
The present municipality Oropos was formed at the 2011 local government reform by the merger of the following 9 former municipalities, that became municipal units (constituent communities in brackets):
The sanctuary for the oracle of Amphiaraus at Oropos is east of Delphi, northeast of Athens
Oropos was founded by colonists from Eretria; it was either located in or identical with Graea. In ancient times, it was a border city between Boeotia and Attica, and its possession was a continual cause of dispute between the two states; but ultimately it came into possession of Athens, and was always an Attic town, even during the Roman Empire. The actual harbour, which was called Delphinium, was at the mouth of the Asopus, about a mile (1.6 km) north of the city.
The famous oracle of Amphiaraus was situated in the territory of Oropus, 12 stadia from the city. The site has been excavated by the Greek Archaeological Society; it contained a temple, a sacred spring, into which coins were thrown by worshippers, altars and porticoes, and a small theatre, of which the proskenion is well preserved. Worshippers used to consult the oracle of Amphiaraus by sleeping on the skin of a slaughtered ram within the sacred building.