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In page Østensjøvannet:

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Østensjøvannet ("Østensjø lake") is named after the Østensjø farm (old Norse Austansjór). The first element is austan 'east of', and the last element is sjór 'sea, lake'. The farm was situated on the lake's east side. The last element is the finite form of vann 'water; lake'. The meaning of the name is then 'the lake belonging to Østensjø'.[1]