Osman Gazi began his first military and political actions on the Byzantine lands. When he became the leader of Kayi Clan in 1281, firstly, he unified the Turkmen clans.

The first war in the Ottoman history was the Armenian-Beli War, in Hamzabey Village 10 km far from Inegol, Bursa (1284). In this battle the nephew of Osman Gazi, Baykoca had died. The first conquest in the Ottoman history, was the Castle of Kulaca in 1285.

The Seljuk Sultan Alaeddin Keykubat III gave Eskisehir city and Inonu region to Osman Gazi. Osman Gazi battled against the Prince of Inegol in 1291 and invaded Karacahisar. He marched through Sakarya. His uncle Dundar Bey was executed because of his connection with the Byzantine Empire.

While Osman Gazi’s political and social progress was continuing, the Ilhans exiled Alauddin Keykubat. The end of the Seljuk Sultanate 1299 is considered as the first year of the Ottoman Sovereign. In the same year the capital was transfered to Bilecik (in some sources 27th July 1301 is accepted as the first year of the Ottomans. In this year the Ottoman Forces defeated the Byzantine army in the War of Befeus and won their independence).

The Castle of Yenisehir was captured in 1300 and the capital moved from Bilecik to Yenisehir the year after.

Osman Gazi divided the lands between his sons, his brother and his arm men, adaquate to the Turkish tradition. He gave Eskisehir to his brother Gunduz, Karacahisar to his son Orhan Gazi, Yarhisar to Hasan Alp, Inegol to Turgut Alp . These were all the Ottoman frontiers. Osman Gazi divided all these strategical areas between his best commanders as he was planing to extand his territories. Commanders as Abdurrahman Gazi, Akcakoca, Samsa Cavus, Konuralp, Aykutalp are the important names in the policy of extension.

Ottomans defeated the Byzantine Army in Koyunhisar in 1302. In 1303, Iznik was invaded and Marmaracik Castle was conquered. The Ottomans were advancing in Anatolia. In the end of Dinboz War in 1306, Kestel, Kete and Ulubat castles were captured and the first military treaty in the Ottoman history was signed. In 1308, Karahisar was conquered and one of important commerce and social centres of the region, Iznik was besieged.

Osman Gazi was a political genius and he began to pressure on the Byzantine Empire by dominating the trade roads. Many Byzantine people even some princes converted to Islam in this period. Kose Mihal the prince of Harmankaya was one of them, he joined the Ottoman Army with his men and castle. He invaded Lefke, Meceke and Akhisar.

In 1315, Bursa was besieged. In 1317, Osman invaded Karatekin, Ebesuyu, Tuzpazarý, Kapucuk and Keresteci castles. In the same year he took Akcakoca and Kocaeli regions.

After 1320, because of age and illness Osman Gazi gave attorney to his son Orhan Gazi. The Ottoman Army under the command of Orhan Gazi invaded Mudanya and Gemlik (1321), Akyazi and Ayankoy (1323), Karamursel and Karacabey (1324), and Orhaneli (1325). Osman Gazi inherited 4800 km2 land and he left 16000 km2 land to his son.

Osman Gazi was establishing new institutions for the Islamisation of the all Ottoman regions. He was a law maker and his laws were mostly based on the Seljuk laws. The first tax was collected in his time. A special kind of tax called “Bac” was impose on the whole sale goods sold in the bazaars. Bac has not been collected from the goods such as a few chickens or a few kilos of oil , sold by the peasants. The Timar system which was in common use by the Selcuks was carried on in Osman Gazi’s time as well. The Sipahis (cavalrymen) were collecting the Timar from the villages they were responsible of and they bought horses, weapons, armours and the other necessary goods for the wars.