|Primary inflows||Tiebel Creek|
|Primary outflows||Seebach to Drava river|
|Max. length||10.7875 km (6.7030 mi)|
|Max. width||1,540 m (5,050 ft)|
|Surface area||10.38 km2 (4.01 sq mi)|
|Average depth||19.6 m (64 ft)|
|Max. depth||52.6 m (173 ft)|
|Water volume||206.28 km3 (167,230,000 acre·ft)|
|Residence time||1.8 years|
|Surface elevation||501 m (1,644 ft)|
|Settlements||Annenheim, Sattendorf, Bodensdorf, Steindorf, Ossiach|
It is situated in the southern Nock Mountains range of the Gurktal Alps along the road from Villach to Feldkirchen, at a height of 501 m (1,644 ft) AA. It reaches a maximum depth of 52.6 m (173 ft), the surface area is about 10.8 km2 (4.2 sq mi).
Ossiacher See is a dimictic lake with mixing periods in spring and in late autumn. In summer the waters reach 28 °C at the surface. Several uninhabited parts of the shore are protected as natural reserves. The main inflow is the Tiebel creek in the eastern bay, where the Bleistatt mire is currently re-naturalised. The lake empties via the Seebach stream to the Drava river.
There are five main villages surrounding the lake, all of them mainly dependent on summer tourism: Annenheim and Sattendorf, both parts of the Treffen municipality, Bodensdorf and Steindorf, all located on the northern shore, as well as Ossiach with Ossiach Abbey, after which the lake is named. The Gerlitzen mountain on the northern shore rises up to 1,909 m (6,263 ft), offering panoramic views over the surrounding ranges. From Annenheim, the Kanzelhöhe subpeak can be reached by cable car, built in 1928 by the Bleichert engineering company.