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 founder of the Ottoman Empire (first known as the Ottoman Beylik or Emirate). While initially a small Turkoman principality during Osman's lifetime, his descendants 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, and 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 reinforce 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.