He and the Volunteers under his command played an important role in the overthrow of Emir Nasr in favour of his nephew Ismail I. Under Ismail, he was appointed the overall chief of the Volunteers (shaykh al-ghuzat) and in this role won a decisive victory against a Castilian Army at the 1319 Battle of Sierra Elvira. His power continued to grew, alienating other ministers in the Emirate, including the Vizier Muhammad ibn al-Mahruq. The struggle between Uthman and Ibn al-Mahruq escalated to a civil war, which ended with Ibn al-Mahruq's assassination on the order of Sultan Muhammad IV and Uthman's retention of his previous power.