 Osiek most likely was a fortified gord, and in the mid-14th century, King Kazimierz Wielki built (or strengthened) a local castle.Osiek was burned in the Mongol invasion of Poland, but the village was quickly rebuilt.In 1430, King Władysław Jagiełło granted it the Magdeburg rights, allowing the town to organize weekly markets on Wednesdays.