Osman I had a powerful general who was Greek and didn't convert to Islam until years after joining Osman's service.
Michael the Beardless, they called him. He was likely a Roman governor who surrendered or defected. His descendants, the Mihaloğlu, were among the three most important aristocratic dynasties in the very early Ottoman state. The other two were the Evrenosoğlu and the Turahanoğlu. The Evrenosoğlu were founded by a man named Evrenos, who also appears to be European (at the very least, his name is neither Turkish nor Persian) and whose father was called the "Frank." The Turahanoğlu were probably Turks, but they appear pretty late in our sources.
Indeed, some early reports from Venetians suggest that the Ottomans were just the biggest of these noble dynasties early on. Supposedly, the four bandit leaders Osman I, Michael, Evrenos, and Turahan realized that they needed to unite into one kingdom to successfully beat the Byzantines, and as Osman was the bravest of them all they chose him as king. The rest was history.
Of course this isn't corroborated in Ottoman sources, which seeks to glorify and legitimize the Ottoman dynasty (hence the famous
Osman's Dream) just as the Venetian source isn't really positive about the Ottomans. But we actually have discovered the tomb of Evrenos dating to 1317 where he is titled "King" in Arabic (
malik), a word otherwise used exclusively for Ottoman sultans. So it's not entirely impossible that people like Michael and Evrenos had more important roles in the very early decades of the Ottoman empire than what's conventionally thought.