The founder and the first sultan of the Ottoman Empire, Osman, was born in 1258 AD in Sogut, near Bursa. He was one of the three sons of Ertugrul Gazi, and his mother was Halime Hatun. Osman Gazi was only 23 years old when he took the control of Kayi Clan in Sogut and than ascended the throne after the death of his father, ruling for 27 years until his death in 1326.

After his ascension to throne, he gathered several Turcoman groups around himself and fought against Byzantines, collaborating with Alaeddin Keykubad I of the Seljuks. When the Seljuk Empire remained rulerless after the exile of Alaaddin, Osman Gazi reunited them under his rule in 1299 marking the history as the foundation of the Ottoman Empire.

Osman Gazi continued his campaigns against the remaining Byzantine Empire in the area along with his brothers, sons, and close supporters. He cut the Byzantine trade routes forcing local rulers to surrender. After 1320 he got sick so his sons continued to fight, capturing many towns from the Byzantines. He died of gout at the age of 68 in Sogut and buried in the city of Bursa.

During his rule, he married Mal Hatun first, from which he had a son, Orhan Gazi who later became the next sultan. After Mal Hatun he married Bala Hatun from which another son, Alaaddin was born who became his Grand Vizier when grown up. He had a total of 8 sons and 1 daughter.

Osman Gazi was nicknamed as Black (Kara in Turkish), which according to an ancient Turcoman tradition signifies brave and heroic person. He was a tall man with a broad and high chest and long arms, dark tan, round face with a thin beard and long thick mustache, ram-nose, hazel eyes, thick eyebrows, had deep and fine voice, expert on horse riding, sword using, and battle strategies. He was fair, helped very much to the poor people, even giving them his own clothes if he saw a person in need.

Besides military actions, Osman Gazi also established all new measures to form a new empire, such as bringing new laws based on Seljuk laws as well, adopting taxes, minting the first Ottoman coins (Akçe), and so on