O'Malley plays Burt Hummel, dad to Chris Colfer's Kurt on the musical. He told the Television Critics Association on Saturday that Monteith was a welcoming colleague, a talented actor and a great guy.
O'Malley said Monteith, who played quarterback Finn, was the quarterback on the set as well.
The 31-year-old Monteith was found dead July 13 in a hotel in Vancouver, British Columbia, and an autopsy revealed he died of an overdose of heroin and alcohol.
O'Malley says the toughest scene he's had to do as an actor was opposite Monteith. On the show, Burt Hummel eventually married Finn's mother.
"I had to throw (his character) out of the house because of a slur that he used. It was remarkable to me when we were shooting that scene over and over again, the depth of emotion he was able to portray."
"Glee" is set to begin its fifth season on Sept. 26, a week later than originally scheduled, and a tribute episode to Monteith is planned.
"I'm here doing 'Welcome to the Family,' and we shoot one episode five days a week. But Burt is a very important role to me. It's been a great part and I've said to all those guys, that I'll work early in the morning, late at night, Saturday, Sunday, whatever I need to do to participate and continue to be on that show honoring Cory and his passing and that character. He is my stepson on that show. I certainly plan on being there."
O'Malley says he believes he will be on hiatus from "Welcome to the Family" when the tribute episode to Monteith is shot, clearing his schedule to appear on the show.