"This was clearly a premeditated breach of all the international aviation rules, regulations, safety," he said.
O'Leary said the pilot was put under "considerable pressure" to land in Belarus instead of the more standard options of Poland or other Baltic countries.
"He wasn't instructed to do so, but he wasn't left with any great alternatives," he told members of the Parliament committee.
After the plane was on the ground, several "unidentified persons" boarded the aircraft with video cameras, according to O'Leary.
They "repeatedly attempted to get the crew to confirm on video that they had voluntarily diverted to Minsk," the Ryanair executive said. The crew refused to provide such confirmation, he said.
Western countries have called the forced diversion a brazen "hijacking" by Belarus. Outraged European Union leaders swiftly slapped sanctions on the country, including banning Belarusian airlines from using the airspace and airports of the 27-nation bloc and telling European airlines to skirt Belarus. UK authorities took similar actions.
O'Leary said he did not support continuing such flight bans in the long term.
"We cannot have a situation whereby airlines, air travel, our customers and our citizens run the risk of being hijacked and diverted under false pretenses," he said. "But equally, far more U.K. citizens will be disrupted as a result of long-haul flights between the UK and Asia, for example, now having to fly around Belarus or avoiding Belarusian airspace."