Osman's origins are extremely obscure, and almost nothing is known about his career before the beginning of the fourteenth century.^[1] The date of 1299 is frequently given as the beginning of his reign, however this date does not correspond with any historical event, and is purely symbolic. By 1300 he had become the leader of a group of Turkish pastoral tribes, through which he ruled over a small territory around the town of Söğüt in the north-western Anatolian region of Bithynia. He led frequent raids against the neighboring Byzantine Empire. Success attracted warriors to his following, particularly after his victory over a Byzantine army in the Battle of Bapheus in 1301 or 1302. Osman's military activity was largely limited to raiding because, by the time of his death, in 1323-4, the Ottomans had not yet developed effective techniques for siege warfare.^[2] Although he is famous for his raids against the Byzantines, Osman also had many military confrontations with Tatar groups and with the neighboring principality of Germiyan.

Osman was adept at forging political and commercial relationships with nearby groups, Muslim as well as Christian. Early on, he attracted several notable figures to his side, including Köse Mihal, a Byzantine village headman whose descendants (known as the Mihaloğulları) enjoyed primacy among the frontier warriors in Ottoman service. Köse Mihal was noteworthy for having been a Christian Greek; while he eventually converted to Islam, his prominent historical role indicates Osman's willingness to cooperate with non-Muslims and to incorporate them in his political enterprise.

Osman I strengthened his legitimacy by marrying the daughter of Sheikh Edebali, a prominent local religious leader who was said to have been at the head of a community of dervishes on the frontier. Later Ottoman writers embellished this event by depicting Osman as having experienced a dream while staying with Edebali, in which it was foretold that his descendants would rule over a vast empire.