In 1680 the lake was called Ölsiön https://www4.isof.se/NAU/bilder/_s2kx001/120207d1/p2/0001299a.pdf
According to SAOL ölle has no meaning as a word in Swedish. https://svenska.se/
Öl in Swedish means beer in English. Öl in German means oil in Swedish.
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mad, maden, mader • sank ängsmark It´s not exactly the same as swamp. You could say it´s somewhere in between a meadowland and a swamp as it is damp. sankmark = fen
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I leave the question of the owls for Kalle to answer. ;)