In 1301 Osman captured Yenisheheer and Youndhissar, making the former the capital of the territory. After besieging Iznick in 1303 and gaining numerous minor victories, he siezed the city of Brusa in 1326. Shortly afterwards, Osman Gazi died.
He was succeeded by his son Orkan Gazi (or Orkan Bey) who continued to extend the boundaries of the new country, adding Izmit and other places to his territories. Orkan gained a notable victory over a Byzantine army which attempted to lift the siege of Nice and added the principality of Karesi to his lands. Angora was regained from the Ahi Tribe and Cheembi Castle, Gallipoli, Bolayir, Malkara, Chorlou and Tekirdagh were added to Ottoman territories.
During the reign of Orkan Gazi coins were used for the first time in the Ottoman Empire. Orkan died in 1360, being succeeded by his son Amurath the First.
Amurath captured Chorlou and Lulebourgaz and regained Angore from the Ahi Tribe once more in 1363. His generals Evranos Bey and Jadji Ilbey annexed Malkara, Keshan, Ipsala, Dedeaghach and Dimetoka while Edirne, Philibe and Gumushhane were also taken.
The Crusader armies attacked Edirne but were defeated by Hadji Ilbey. After further victories the Bulgarian Kingdom was annexed to the Ottoman Empire and Amurath married Maria, the sister of the Bulgarian King.
After the Serbian Tribe was defeated in 1371 their leader acknowledged the overlordship of the Ottoman Empire and
agreed to pay 50 okkas of silver to the Sultan. He also agreed to send troops to fight for the Empire as and when needed. Following this victory Amurath returned to Brusa and married his son, Bayezid, to Solyman Shah's daughter, receiving Kutalya, Tavshanh, Simav and Emet as dowry.
Aksheheer, Karaaghach, Eghirdir and Hamidogllou were purchased from Hussein Bey and, in 1385, Ishtip, Monaster and Ohree were conquered by Timourtash Pasha. Sophia and Nish in Bulgaria became mandates of the Ottomans but there was a setback when the Kings of Serbia and Bosnia and the Princes of Albania and Crotia defeated Timourtash Pasha at Ploshnik with an army of 30,000 troops.
In an attempt to take advantage of this reversal, several European nations formed a Union of Crusaders, but before they could launch an assault the armies of the Ottoman Empire, under Ali Pasha, defeated the forces of the King of Bulgaria and the Prince of Dobroudja, thus preventing a Crusader attack. Amurath then passed on into Roumelia and in 1389 put the Crusaders to rout.
Tragically, Amurath was slain by a Serbian soldier after the battle and was succeeded by his son, Bayezid.
Tribes such as the Menteske and Hamid Oghoullari seized the opportunity to declare war on the Ottoman Empire but Bayezid the Yilderim (Lightning) quickly moved against them and put an end to their challenge. Beysheheer was ceeded to the Empire and peace was declared.
Sultan Bayezid Khan now besieged Istanbul, an action which led to a new Crusade. At the Battle of Nighbolou the Crusaders were utterly defeated and the siege of Istanbul continued. The Anatolian Castle was built and Bayezid, leaving the siege in the hands of the Vizier Ali Pasha, passed on to Anatolia and annexed Koniah. Burhanuddin and Malatia were also conquered.
While Bayezid was away, a fleet under the command of Boucicant raised the siege of Istanbul and regained the castles. Bayezid renewed the siege in 1400 but the invasion of Anatolia by Timour caused him to lift it again.