Osman I (1258–1326) (Ottoman عُثمَان ʿUthmān) was the founder of the Ottoman Empire. He was born in 1258 and inherited the title bey (chief) from his father, Ertuğrul, as the ruler of the village of Söğüt in 1281. The birth of the empire originated with the conquest of the Turkish tribe of Eskenderum and the city of Eskişehir (Turkish for 'Old Town') in 1301–1303, although Osman had already in 1299 declared the independence from the Seljuk Empire of his own small kingdom, the Ottoman Principality.

With the fall of the Byzantine fort at Yenişehir ('New Town') the Turks where ready to launch a siege at the large Byzantine towns of Proussa (now Bursa) and Nicaea (now İznik). Osman I died in 1326, the same year that Bursa fell. He was after his death given the title of ghazi (warrior of the faith) by his successors. Whenever a new Sultan ascended the throne, the people would cry out "May he be as great as Osman".