The Ottoman Empire (/ˈɒtəmən/; Ottoman Turkish: دَوْلَتِ عَلِيّهٔ عُثمَانِیّه‎ Devlet-i Aliyye-i Osmâniyye, Modern Turkish: Osmanlı İmparatorluğu or Osmanlı Devleti), also known as the Turkish Empire, Ottoman Turkey or Turkey, was an empire founded in 1299 by Oghuz Turks under Osman I in northwestern Anatolia. After conquests in the Balkans by Murad I between 1362 and 1389, the Ottoman sultanate was transformed into a transcontinental empire and claimant to the caliphate. The Ottomans ended the Byzantine Empire with the 1453 conquest of Constantinople by Mehmed the Conqueror.
Conquests by who began the transformation of the Ottoman sultanate into an Empire?
Murad I