Meanwhile fares fell 3 per cent amid fierce competition as the total number of available seats grew across Europe.
But Ryanair still managed to grow pre-tax profits in the 12 months to March by 10 per cent to €1.3 billion ($2.2b) thanks to a boom in passenger numbers that pushed up sales 8 per cent.
Sales were boosted by a 13 per cent surge in so-called "ancillary" revenues such as luggage fees, reserved seats and car hire, which now account for around one third of the carrier's top line.
O'Leary warned that costs were likely to rise 9pc this year as Ryanair contends with a higher wage bill and the soaring price of oil, which the airline expects to add €400 million to its fuel bill.
A fierce critic of Britain's decision to leave the European Union, O'Leary also cautioned that UK shareholders in Ryanair could find their voting rights restricted in the event of a hard Brexit to comply with rules that dictate EU airlines should be majority owned by investors within the bloc.
Ryanair is currently awaiting approval from EU regulators for its purchase of a 75 per cent stake in LaudaMotion, the low-cost Austrian airline backed by former Formula 1 star Niki Lauda.
Shares in Ryanair were up 3.5 per cent at €16.04 in early trade.