I can think of 5 American zoos and 3 Canadian zoos with the species:
Alaska Wildlife Conservation Centre
Alaska Zoo
Assiniboine Park Zoo
Calgary Zoo
Greater Vancouver Zoo
Hemker Park & Zoo
Minnesota Zoo
Point Defiance Zoo
Then there is this herd of 9 Musk Ox at Rolling Hills Wildlife Adventure:
In Alaska you also have a captive herd at the Large Mammal Research Center in Fairbanks and at the Musk Ox Farm in Palmer. Both facilities are open to visitors.