Pat O’Brien and his mustache never really left. Since quitting “The Insider” two years ago, he’s been a “social commenter” on several TV shows and even filled in for Don Imus — but now he’s back to what he loves most. On Monday, O’Brien joins Steve Hartman and Vic “The Brick” Jacobs on Fox Sports Radio’s “Loose Cannons” program. The show, previously known as “Hartman & Friends,” airs weekday afternoons. “It’s going to be fun,” O’Brien, who worked 16 years for CBS Sports, told Page Six yesterday. “A day doesn’t go by when someone doesn’t ask, ‘When are you going back to sports?’ This feels like going back home.” The native of Sioux Falls, SD, will be broadcasting from a studio in Burbank. “I’m grateful for the opportunity,” he said. “There are second chances in this world, and I’m living proof.”