Orhan Gazi was a very brilliant soldier. His most important achievement was the invasion of Bursa on 6th April 1326. Orhan Gazi appointed Alaeddin Ali Bey as vizier whom captured the Province of Orhaneli, came to Bursa and besieged the city. Bursa was taken under control without an important skirmish. Afterwards, the centre of the state was transfered to Bursa from Bilecik.

On the other hand the famous commanders like Akcakoca, Karamursel and Abdurrahman Gazi took the castles of Kandra, Aydos, and Semendre. Thus, the territories of the Ottoman state were extended through Bosphorus and the Black Sea. Orhan Gazi spent his life in battle fields and won many victories, he organised a new troop called “Yaya”(infantry), in 1328.

The Ottomans captured the castles in Kocaeli Peninsula and came to the Bosphorus. Byzantine Emperor Andronikos III organised an army to take back the castles and to protect Iznik that was under siege. Orhan Gazi left some of his forces in Iznik and advanced through the Byzantine Army. Two armies came across in Palekonon (Maltepe) the region that named this battle. The Ottoman Army had defeated the Byzantine army (1329).

After this important victory Orhan Gazi returned to Iznik to continue to blocade the city. The commander of Iznik had lost his hope from the Byzantine Empire and claimed that he would have conditionally surrendered. Orhan Gazi had accepted the Commander’s conditions and commanded his soldiers to be kind to the people of Iznik. Thus, one of the most important cities of the Christian World was annexed to the Turkish sovereignty (1330).

Orhan Gazi continued his campaigns and he invaded Tarakli in 1331, Goynuk, Mudurnu, and Gemlik in 1333. Orhan Gazi conquered the important trading centre Izmit and its province (Koyunhisar, Hereke, Yalova, Armutlu ) in 1337. He gave the administration of the region to his son Suleyman Pasha.

In 1342 the castles of Kirmasti, Mihalic and Ulubat (Balękesir province) were conquered. With these conquests, the Ottomans had been neighbours with the Principality of Karesiogullarę. Orhan Gazi conquered the Karesiogullarę by benefiting from their throne struggles (1345). Famous commanders as Haci Ilbey and Evrenus Gazi from Karesiogullarę joined the Ottoman Army moreover the navy of the principality was captured as well. The Marmara Islands, Uskudar and Kadikoy (Istanbul province) were conquered (1352). In 1354, the principality of Gerede was invaded and Ankara was conquered for the first time.