It was the Seljuk Turks who defeated the Byzantines at Manzikert around the late 11th century, close to where Armenia is. However, within 4 centuries, on 1453, to be precise, they become Ottomans by the time they conquer Constantinople.
What were the reasons that they changed their title? Was it because their ethnicity slowly changed from Central Asians into Middle Easterners, and their title also changed with this gradual transition?
"Ottoman" is named after Osman I, the son of Ertuğrul the Turkik leader who gains control of the lands around The Bursa/Iznik region (on the South shore of the Marmara Sea). Ertuğrul's tribe enters Anatolia and helps the Seljuks wrest that area from the Romans. Ertuğrul Dies shortly after winning the land.
"Documentation of the early history of the Ottomans is scarce. According to semilegendary accounts, Estugrul, khan of the Kayi tribe of the Oguz Turks, fled from Persia in the mid-thirteenth century to escape the Mongol hordes and took service with the sultan of Rum at the head of a gazi force numbering "400 tents." He was granted territory--if he could seize and hold it--in Bithynia, facing the Byzantine strongholds at Bursa, Nicomedia (Izmit), and Nicaea. Leadership subsequently passed to Estugrul's son, Osman I (reigned ca. 1299-1326), the eponymous founder of the Osmanli dynasty--better known in the West as the Ottomans-- that was to endure for 600 years."
Oğuz Turks and Seljuk Turks are no the same, but related tribes.