Öljaren, Lake in Södermanland County, Sweden

Öljaren is a freshwater lake in Södermanland County that extends across several kilometers and forms boundaries between local municipalities. The shore includes wooded areas and open stretches with multiple access points scattered around its perimeter.

The name Öljaren comes from the Old Swedish word Yli, meaning "raging", in the genitive Ylia, later altered to Ölja(n) and then to Öljaren under the influence of nearby lake Hjälmaren. The lake drains north through the 1.5 km long Forsån into Hjälmaren. On its eastern shore lie Julita gård, Julita kyrka and Gimmersta herrgård, while the Forsby estate stands on the western side.

The name comes from the Old Swedish Yli, "raging" (genitive Ylia), later reshaped under the influence of nearby Hjälmaren into Ölja(n) and then Öljaren. Two melodies, "En afton på Öljaren" and "Vid Öljarns sköna strand", were composed by Philip Widén. On the eastern shore stand Julita gård, Julita kyrka and Gimmersta herrgård; the Forsby estate lies on the western side.

A bathing spot lies on the western shore at Millerstu, and the lake drains north through the 1.5 km Forsån stream into Hjälmaren. The water is very nutrient-rich and eutrophication is a problem, with fish kills from oxygen shortage reported, so check conditions before swimming.

Several small uninhabited islands scattered across the lake serve as nesting grounds for native bird species. These sheltered refuges host populations of waterfowl and marsh birds that remain hidden from activity along the shores.