ボレスワフ4世
Osielsko is mentioned in Ptolemy's Geography as "Askaukalis", settlement of the Goths. In the Middle Ages, the region was inhabitated by the Polish tribes, and in the 10th century it became part of the emerging Polish state. In 1065, it was granted to the Mogilno monastery. In 1155, Osielsk was mentioned in a document of Polish monarch Bolesław IV the Curly.