Final answer:
The major rival of the Ottoman Empire before 1450 was the Byzantine Empire.
Step-by-step explanation:
The major rival of the Ottoman Empire before 1450 was the Byzantine Empire. The Ottomans conquered Constantinople in 1453, which was the capital of the Byzantine Empire. The Byzantines had managed to survive and thrive culturally and economically for centuries, but were eventually surrounded by the Ottomans and Constantinople was ultimately captured by the Ottomans.