Mieszko's son Casimir I of Opole, Duke from 1211, invited German settlers immigrating to his duchy in the course of the Ostsiedlung, and granted German town law to settlements like Leśnica, Ujazd, Gościęcin, Biała and Olesno.
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The town was first mentioned in a border agreement between Władysław Opolski, the duke of Opole and Racibórz and Ottokar II of Bohemia, in 1256 as Bogun (today's Old Bohumín).