The plane, carrying 168 people, diverted Monday night to Limoges, France, after the cabin pressure dropped suddenly, said Michael O'Leary, Ryanair's chief executive.
Several passengers on the flight, including Arctic explorer Pen Hadow, whose 9-year-old son, Wilf, was hospitalized, said oxygen masks failed to work properly and the crew failed to calm frightened passengers.
"Well over 80 percent of people on that flight knew they were going to die," Hadow told The Times of London.
O'Leary, however, said the masks did function and the crew acted properly. The crew, he said, couldn't communicate with the passengers because safety rules require the pilots and cabin staff to put oxygen masks on immediately.
The 16 people hospitalized were being treated for ear problems caused by the sudden decompression, authorities said.