He warned people to expect major power outages that could last for 72 hours.
"This is the real deal. This is a big, deadly and dangerous storm," O'Malley said.
The director of Maryland's Emergency Management Agency, Richard Muth, said Irene "could certainly be a catastrophic event."
Craig Fugate, administrator of the Federal Emergency Management Agency, urged those in Irene's projected path to check if they live in an evacuation zone and to stock up on supplies.
"If you're still in denial, it's too late. You need to get ready. I do expect a lot of trees down, a lot of power outages, a lot of flooding, so now is the time to get ready for those types of impacts," Fugate said. "Get your supplies, make sure you have plenty of batteries. If the power goes out, especially with all of these trees, it may take them days or longer to get the power back up."
ABC News' Matt Hosford, Dan Harris, Steven Portnoy and the Associated Press contributed to this report.