Jet fuel costs will rise as a result of the war in the Middle East, and airlines will pass the increase on to ticket prices, according to O'Leary.
"I think that over the next twelve months you will see the biggest increase in airline ticket prices in the history of Europe," he said at a press conference in Brussels.
Airlines usually hedge against the risk of higher jet fuel prices with long-term supply contracts, but signing new long-term contracts for future deliveries has now become expensive.
He added that aviation taxes and regulations are also pushing up costs for the industry.
Ryanair has already lowered its passenger forecast for the winter season because of the situation.