Excerpts from Books and Wikipedia
"As the dynasty [Seljuk Empire] declined in the middle of the thirteenth century, the Mongols invaded Anatolia in the 1260s and divided it into small emirates called the Anatolian beyliks. Eventually one of these, the Ottoman, would rise to power and conquer the rest." [Wikipedia: Seljuk Empire]
"Osman I or Osman Gazi (...), sometimes transliterated archaically as Othman, was the leader of the Ottoman Turks and the founder of the Ottoman dynasty. He and the dynasty bearing his name later established and ruled the nascent Ottoman Empire (then known as the Ottoman Beylik or Emirate). The state, while only a small principality during Osman's lifetime, transformed into a world empire in the centuries after his death." [Wikipedia]