The cost of free ticket vouchers issued to passengers affected in the new round of cancellations will be about 25 million euros ($29 million), bringing the tally since the start of the crisis to nearly 50 million euros.
Mr O'Leary pledged: "From today, there will be no more rostering-related flight cancellations this winter or in summer 2018".
Additionally, Ryanair said nearly 40% of its annual leave requirement for 2018 during the first quarter, which it expects will remove the risk for recurring pilot rostering problems next year.More news: Backed Militia Says It Has Recaptured Syria's Biggest Gas Field from ISIS
Ryanair will operate 25 fewer planes than planned during its winter schedule starting in November, and then 10 fewer from April, it said Wednesday.
The latest announcement will affect 34 routes, including London-Belfast and Hamburg-Oslo.
By reducing its flying schedule, the Dublin-based carrier said it will eliminate all risk of further flight cancellations, because slower growth will create "lots of spare aircraft and crews" across Ryanair's 86 bases this winter.
"We have less than 400,000 customers booked on these flights and many of these flights have zero bookings at this time", a statement from Ryanair said.More news: NBC 'Sunday Night Football' ratings lowest for Week 3 since 2006
Just two weeks ago, the company was forced to ground up to 50 flights a day over a period of six weeks because it "messed up" pilots' holidays, with the cancellations affecting up to 9,000 travellers a day.
Ryanair says it will offer affected passengers alternative flights or full refunds.
See our Ryanair cancels 100s of flights news story for a full rundown of your rights and what you can try if you're affected.
The flight cancellations from last week are expected cost the company less than 25 million euros ($30 million) and the free flight vouchers issued to affected passengers will also be less than 25 million euros, the company said.More news: Trudeau won't weigh in on Kurdish independence, citing Quebec lesson
"We have notified the Alitalia bankruptcy Commissioners that we will not be pursuing our interest in Alitalia or submitting any further offers for the airline", Ryanair said.