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Oresthasium was a village in ancient Arcadia, south-east of Megalopolis. It was, according to myth, founded by Orestheus, King of Arcadia, but it was also mythologically connected to Orestes. In historical times, it formed part of the territory of Maenalus, but, upon the foundation of Megalopolis in 371 BC, Oresthasium was abandoned and incorporated into Megalopolis.