His trip came as two former stewards prepared to sue the Irish carrier for unfair dismissal and breaching labour laws.
"It was a contract of slavery," said Vegard Einan of the Parat union that is assisting the two women in their lawsuit.
O'Leary, speaking to reporters at Rygge airport, said that his firm fully respected European legislation and that he was the victim of "a series of false claims".
The airline is the target of growing criticism in Norway, where politicians and unions are calling for travellers to boycott it because of its contested labour practices.
On Wednesday, Norway's Prime Minister Jens Stoltenberg declared that he would never travel with Ryanair, a no-frills airline which makes most of its money by charging for optional extras.