Upon Osman's death his son Orhan succeeded him as leader of the Ottomans. Orhan oversaw the conquest of Bithynia's major towns, as Bursa (Prusa) was conquered in 1326 and the rest of the region's towns fell shortly thereafter.^[2] Already by 1324, the Ottomans were making use of Seljuk bureaucratic practices, and had developed the capacity to mint coins and utilize siege tactics. It was under Orhan that the Ottomans began to attract Islamic scholars from the east to act as administrators and judges, and the first medrese (University) was established in Iznik in 1331.^[3]

In addition to fighting the Byzantines, Orhan also conquered the Turkish principality of Karesi in 1345-6, thus placing all potential crossing points to Europe in Ottoman hands. The experienced Karesi warriors were incorporated into the Ottoman military, and were a valuable asset in subsequent campaigns into the Balkans.

Orhan married Theodora, the daughter of Byzantine prince John VI Cantacuzenus. In 1346 Orhan openly supported John VI in the overthrowing of the emperor John V Palaeologus. When John VI became co-emperor (1347–1354) he allowed Orhan to raid the peninsula of Gallipoli in 1352, after which the Ottomans gained their first permanent stronghold in Europe at Çimpe Castle in 1354. Orhan decided to pursue war against Europe, Anatolian Turks were settled in and around Gallipoli to secure it as a springboard for military operations in Thrace against the Byzantines and Bulgarians. Most of eastern Thrace was overrun by Ottoman forces within a decade and was permanently brought under Orhan's control by means of heavy colonization. The initial Thracian conquests placed the Ottomans strategically astride all of the major overland communication routes linking Constantinople to the Balkan frontiers, facilitating their expanded military operations. ln addition, control of the highways in Thrace isolated Byzantium from direct overland contact with any of its potential allies in the Balkans and in Western Europe. Byzantine Emperor John V was forced to sign an unfavorable treaty with Orhan in 1356 that recognized his Thracian losses. For the next 50 years, the Ottomans went on to conquer vast territories in the Balkans, reaching as far north as modern-day Serbia. In taking control over the passageways to Europe, the Ottomans gained a significant advantage over their rival Turkish principalities in Anatolia, as they now could gain immense prestige and wealth from conquests carried out on the Balkan frontier.