The founder of the Ottoman Empire, Osman Gazi (or, as he is known, Osman Khan or Osman Bey) was descended from a line of great leaders who had, in turn, led the Kayi Tribe, the most famous of all the 24 Turkish tribes.
Osman Khan's father, Ertoghrul Gazi, had been appointed Uchbey on the Byzantine Frontier by the Seljukian Sultan, Alauddin. The land was given to him to control and lay along the boundaries of Brusa, Kutahya and Biledjik. Ertoghrul Gazi captured the town of Saegut from the Byzantine Empire and made it the capital city of the region. The duties of the Uchbey were to defend the frontiers of the Empire and to fight against the attacks of the Crusader Knights.
On the death of Ertoghrul Gazi in 1281, his son Osman Gazi, despite being the youngest member of his family, was elected Uchbey to succeed his father.
Osman Gazi, through a clever mixture of diplomacy and warfare, gained large tracts of land from the neighboring Byzantine Emperors. Faced by an alliance of the Byzantine Emperors of Brusa and Nice on the one hand, and Yarhisar and Karadjahisar on the other, Osman Gazi declared war. He attacked Nice and in 1291 captured Karadjahisar. He changed the Castle Church into a mosque and assigned a judge to rule the area.
After further victories at Biledjik and Yarhisar in 1299 he married his son, Orkan Gazi, to the daughter of the Byzantine Emperor of Yarhisar.
In 1299 Osman Gazi declared independence, granting fiefs to his Moslem army veterans and appointing wardens, judges and magistrates to take charge of castles and strong points.