Who founded/started the Ottoman Empire?
It is typically not possible to say who founded a given empire. This is largely because we cannot really say that a political entity becomes an empire at some given point where it was not an empire the day before. However, if there were one person who could be called the founder of the Ottoman Empire, it would be Osman I. His name is where the Ottoman Empire got its name.
Osman was the ruler of a small state in Anatolia (also called Asia Minor). As the Byzantine Empire weakened, Osman expanded his holdings. He is said to have had a vision of creating an empire with himself as ruler. Under Osman, the Ottomans expanded their territory greatly. For this reason, he is typically seen as the first ruler of the Ottoman Empire.