There were plans to produce a sequel, with the main characters on college spring break. Upon reading the script, Will Ferrell when promoting Semi-Pro (2008), said that he and Vince Vaughn had the same reaction when the story just felt like it was repeating itself.
Frank's streaking scene was shot on a city street. As Will Ferrell remembered it, one of the storefronts was a 24-hour gym with Stairmasters and treadmills in the window. "I was rehearsing in a robe, and all these people are in the gym, watching me. I asked one of the production assistants, 'Shouldn't we tell them I'm going to be naked?' Sure enough, I dropped my robe and there were shrieks of pure horror. After the first take, nobody was at the window anymore. I took that as a sign of approval."
Will Ferrell was really naked for Frank's streaking scene. He justified it by saying it showed his character falling off the wagon. "The fact that it made sense was the reason I was really into doing it, and why I was able to commit on that level," he explained to the BBC. "If it was just for the sake of doing a crazy shot, then I don't think it makes sense." Still, Ferrell needed some liquid courage, and was intimidated by the presence of Snoop Dogg.
The song "Here I Go Again" by Whitesnake is played four times in the movie - twice with the words, once without, and once hummed. The band Whitesnake is also mentioned by Mitch when he references Nicole's high school jean-jacket. Frank also wears a Whitesnake t-shirt at the very end of the movie, during the credits.
Snoop Dogg agreed to cameo in the film so he could play Huggy Bear in Starsky & Hutch (2004). Todd Phillips admitted to essentially bribing him, using his desire to play Huggy Bear to his advantage. "So when I went to him I said, 'I want you to do Huggy Bear,' he was really excited. And I said, 'Oh yeah, also will you do this little thing for me in Old School a little cameo?' So he kind of had to do it I think."
Vince Vaughn and his friends accepted an invitation to hang out in Snoop Dogg's trailer to play video games on the last day of shooting. Vaughn recalled seeing Luke Wilson later watching the news alone in his trailer; he had not been informed of the get-together.
Terry O'Quinn agreed to play Goldberg, uncredited, in what was a two-day job for him. He neglected to inform his sons he was in the movie, and when they saw it, one of them called their father. "I got a call from my sons one night, and they said, 'What were you doing in Old School? We didn't even know you were in it!' They said, 'We're sitting there, and the first time we see you, it's, like, in a reflection in a window. And when we saw it, and we both thought we were, like, tripping or something!'"
Rob Corddry had a jewel bag around his private parts for his nude scene, but his butt made it into the final cut. He had to sign a nudity clause, which gave the film the right to use his naked image "in any part of the universe, in any form, even that which is not devised."
Before filming, Vince Vaughn worked with Will Ferrell to figure out their characters' backstories and how they knew each other; he credited that with helping him figure out who Bernard was, which led to several ad-libbed moments. "The earmuff scene where he swears in front of the kids, and then I tell the kid to earmuff, that all is off the cuff. But that stuff is a lot easier to do when you know who you are and your circumstances, and who your characters are," Vaughn explained.
The idea for the film came when Todd Phillips was talking to a friend of his who had seen and enjoyed Phillips' movie Frat House (1998) and told him, "You know what would be funny is a movie about older guys who start a fraternity of their own." After being told by Phillips to write it, he presented Phillips with a "loose version" of the finished product.
Vince Vaughn, Luke Wilson, and Will Ferrell dubbed themselves "The Wolfpack"-years before Todd Phillips directed The Hangover (2009)-because they would always make fun of each other. A particularly stinging exchange had Ferrell refer to Legally Blonde (2001) (which Wilson had co-starred in) as Legally Bland. Wilson said it didn't make him feel great. Wilson retorted by saying to Ferrell, "Will, you know the transition from TV to the movies isn't a very easy one, so you might just want to keep one foot back in TV just in case this whole movie thing falls through!"