Prime Minister Calvin Armstrong Rwomiire Akiiki announced the death on Friday, calling it an "immeasurable loss" to the royal family and the people of Tooro. He urged the kingdom's subjects to "remain united, calm and prayerful during this profoundly difficult time."
The Tooro kingdom did not disclose the cause of King Oyo's death, but a Ugandan government minister was quoted in local media as saying that the king had been critically ill and was seeking treatment in the United States.
Oyo was listed by Guinness World Records as the world's youngest reigning monarch. He was crowned upon reaching adulthood at age 18 in 2010 and served as the 13th monarch of a kingdom founded in the early 19th century.
Uganda is governed by an elected president and parliament, but several traditional kingdoms continue to exercise significant cultural influence despite having no formal political power. King Oyo's reign was seen as a stabilizing cultural force in the region.