Final answer:
The Byzantine Empire was the major rival of the Ottoman Empire before 1450, while the Habsburg Empire became a prominent rival after 1450.
Step-by-step explanation:
Before 1450, the major rival of the Ottoman Empire was the Byzantine Empire, which was eventually conquered by the Ottomans in 1453. After 1450, the Ottomans faced several major rivals, but one prominent rival was the Habsburg Empire. This rivalry lasted for several decades and reached a stalemate, with both powers vying for control in Central Europe.