Most sources agree that the Ottoman Turks belonged to the Kayı Oghuz Turkic clan, who, according to Ottoman traditions, fled their native homeland in Central Asia, during the early 13th century, due to the Mongol invasions. The clan settled in Anatolia, in a region belonging to the Seljuk Sultanate of Rûm. According to several sources, the Kayı warriors were known for filling first lines in battles, and their fighting skills and bravery were among the major factors the Seljuks were victorious in many battles. This fact prompted Sultan Kayqubad to appoint Ertuğrul, the clan's Emir, as a Moqaddam (Lieutenant), and to reward the Kayıs some fertile lands near Ankara, where they settled and remained in the service of the Sultan for several years. Later, Ertuğrul was granted dominion over the town of Söğüt in northwestern Anatolia on the Byzantine frontier. He started raiding the Byzantine possessions in the name of the Sultan successfully conquering several towns and villages, and slowly expanding his dominion during the half of the century he spent as a Seljuk governor. In 1281 CE, Ertuğrul passed away at nearly 90 years of age.

Although the exact date of Osman's birth is unspecified, some sources indicate that he was born on 13 February 1258 CE, the exact same day the Mongol hordes invaded Baghdad, killing its inhabitants and ravaging its landmarks. The few available sources agree that he was born in the town of Söğüt, which his father Ertuğrul took as the capital of his emirate.

Osman's last campaign was against the city of Bursa. Although Osman did not physically participate in the battle, the victory at Bursa proved to be extremely vital for the Ottomans as the city served as a staging ground against the Byzantines in Constantinople, and as a newly adorned capital for Osman's son, Orhan. According to some sources, Osman passed away just before the fall of the city, while others suggest that he lived long enough to hear about the victory on his death-bed.

Osman is considered the founder of the Ottoman dynasty, He started an imperial line that will expand to include thirty-five sultans, who ruled one of the largest and mightiest empires in history. The Ottoman Empire will last till 1918 CE, where it collapsed alongside the other Central Powers, after their defeat in the First World War. Osman is often referred to as the first in the line of Ottoman Sultans, although he himself never carried this title in his life, and was instead called "Bey" or "Emir".