Osman Ghazi the first Sultan of Ottoman Empire

 Osman I or Osman Ghazi (Ottoman Turkish: عثمان غازى‎, romanized: ʿOsmān Ġāzī; Turkish: I. Osman or Osman Gazi; died 1323/4), sometimes transliterated archaically as Othman, was the leader of the Ottoman Turks and the founder of the Ottoman dynasty. The dynasty bearing his name later established and ruled the Ottoman Empire (first known as the Ottoman Beylik or Emirate). This state, while only a small Turkmen principality during Osman's lifetime, transformed into a world empire in the centuries after his death. It existed until shortly after the end of World War I.

Owing to the scarcity of historical sources dating from his lifetime, very little factual information about Osman has survived. Not a single written source survives from Osman's reign. The Ottomans did not record the history of Osman's life until the fifteenth century, more than a hundred years after his death. Because of this, historians find it very challenging to differentiate between fact and myth in the many stories told about him. One historian has even gone so far as to declare it impossible, describing the period of Osman's life as a "black hole".

According to later Ottoman tradition, Osman's ancestors were descendants of the Kayı tribe of Oghuz Turks However, many scholars of the early Ottomans regard it as a later fabrication meant to shore up dynastic legitimacy.

The Ottoman principality was one of many Anatolian beyliks that emerged in the second half of the thirteenth century. Situated in the region of Bithynia in the north of Asia Minor, Osman's principality found itself particularly well-placed to launch attacks on the vulnerable Byzantine Empire , which his descendants would eventually go on to conquer.

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 13^th century, due to the Mongol invasions. The clan settled in Anatolia, in a region belonging to the Seljuk Sultanate of Rûm. Other sources claim that the Kayı clan moved to Anatolia two centuries earlier than the previously mentioned date, alongside the Seljuks, when they left Transoxiana to Khurasan around 1040 CE to reside near the city of Merv. Then, the Kayı clan moved towards eastern Anatolia after 1071 CE, where it displaced alongside other Turkic clans. Later, it became involved in the army of Sultan Kayqubad I and fought against the Khawarizmians, Mongols and Byzantines, who were raiding Seljuk lands. 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 Sogut in northwestern Anatolia on the Byzantine frontier. He also Obtained the title of Uç beyliği or Uc bey (literally: marcher-lord). Granting this title was in line with the traditions of the Seljuk Sultanate, which is rewarding any clan chieftain who rises to power and is joined by a number of smaller clans, the title of marcher-lord. However, Ertuğrul had far-reaching political ambitions. He sought to expand beyond the lands rewarded to him. Thus, 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 680 AH / 1281 CE, Ertuğrul passed aways at nearly 90 years of age.