The aid worker, Olivier Vandecasteele, was flown late Thursday from Tehran to Muscat, the capital of Oman, where the exchange took place, Belgium’s prime minister, Alexander De Croo, said on Friday.

In Belgium, a massive campaign called “Free Olivier Vandecasteele” advocated for his return, attracting wide media attention and with banners and posters put up across the country. Vandecasteele said ...