Orhan Osman oglu, also called Orhan Gazi also spelled Orkhan, (born 1288—died 1360), the second ruler of the Ottoman dynasty, which had been founded by his father, Osman I. Orhan’s reign (1324–60) marked the beginning of Ottoman expansion into the Balkans.

Under Orhan’s leadership, the small Ottoman principality in northwestern Anatolia continued to attract Ghazis (warriors for the Islamic faith) from surrounding Turkish emirates fighting against Byzantium. In 1324 the Byzantine town of Brusa (later Bursa) fell to the Ottomans, followed by Nicaea (modern İznik) in 1331 and Nicomedia (modern İzmit) in 1337.
Turning to the neighbouring Turkmen states, Orhan annexed the principality of Karası, which had been weakened by dynastic struggles (c. 1345), and he extended his control to the extreme northwest corner of Anatolia. In 1346 the Ottomans became the principal ally of the future Byzantine emperor John VI Cantacuzenus by crossing over into the Balkans to assist him against his rival John V Palaeologus.

As John VI’s ally, Orhan married Theodora, John’s daughter, and acquired the right to conduct raids in the Balkans. His campaigns provided the Ottomans with an intimate knowledge of the area, and in 1354 they seized Gallipoli as a permanent foothold in Europe.

Orhan’s reign also marked the beginning of the institutions that transformed the Ottoman principality into a powerful state. In 1327 the first silver Ottoman coins were minted in Orhan’s name, while the Anatolian conquests were consolidated and the army was reorganized on a more permanent basis. Finally, Orhan built mosques, medreses (theological colleges), and caravansaries in the newly conquered towns, particularly the Ottoman capital, Bursa, which later became a major Islamic centre.