He said Ryanair would keep seats filled by lowering fares an average 9 percent in the current quarter and potentially more in the first quarter of 2014.
Ryanair's first-half results offered no surprises. Sales for the April-September period rose 5 percent to 3.26 billion euros as the airline carried 49 million passengers, a 2 percent gain. Net profits rose 1 percent to 602 million euros.
The forecast means Ryanair expects to record second-half losses of up to 100 million euros.
Ryanair also announced it would introduce fully assigned seating in February, a significant cultural change for the company that reflected heavy customer criticism of existing policies.
For decades Ryanair had required passengers to form lengthy queues to maintain position for an at-times unseemly scramble for unassigned seats. That practice saved Ryanair costs but meant couples, families or groups sometimes couldn't sit together.
O'Leary said passengers wishing to book a particular seat would be charged 5 euros ($6.75) each, while all others would be assigned seats for free the day before the flight.
O'Leary said he expected Ryanair to resume expansion as it starts to receive the first aircraft from a blockbuster 175-plane deal with sole supplier Boeing. "High-cost competitor airlines are continuing to cut capacity in major markets such as France, Germany, Poland, Spain and Italy, and this continues to create growth opportunities for Ryanair," he said.