Oskjuvatn is a large crater lake in the Askja caldera in the Dyngjufjoll mountains, north of Vatnajokull glacier. Oskjuvatn is Iceland's second-deepest lake.
The depth of Oskjuvatn is 217 m (712 feet) and it has a surface area of 11 km². The lake was formed in the volcanic eruption of 1875, at the same time as the Viti crater. Askja's latest eruption was in 1962.