Prepping for an O Positive reunion requires a bit of heavy training, says frontman Dave Herlihy. “It’s like being Rip van Winkle — you wake up and everything feels stiff. But then you might go for a jog and you’re saying ‘Hey, I can do this’– you might start with 5K, but you can’t roll right out of bed and run a marathon. So it takes some time to get the rust off, but the collective ingredients are still there. It’s gonna be us and it’s gonna be a blast.”

One of Boston’s favorite bands in the ‘80s and ‘90s, O Positive has only played a couple reunion shows on special occasions, like the closing of TT the Bear’s Place in 2014. But this Friday at the Paradise they’ll be doing it for a friend: A close associate of the band was recently diagnosed with brain cancer, so the show was designed to give her both financial and moral support. The band’s classic lineup (Herlihy, lead guitarist Alan Petitti, bassist Dave Ingham, drummer Ken Hickey and utility man Dave Martin) is all aboard. Also reuniting for the occasion are Three Colors, the sophisticated pop band that includes frontman Chris Harford, later a solo artist, and saxophonist Dana Colley (later of Morphine)).

O Positive’s music usually landed on the artful side, and a few like-minded bands (including the Pixies and Throwing Muses) opened for them early on. Their local breakthrough “With You” had a moody Brit-pop sound; but the followup “Talk About Love” was funky and upbeat, and the later single “Back of My Mind” was a garagey rocker. Says Herlihy, “Our MO was always ‘Is this interesting, do I like it?’ And if it was, we did it. In 1985 I was listening to WZBC, and nobody else in Boston was doing the Cure-type modern rock thing. Each record we did is a reflection of where we were at the time. It can sound pretty odd going from one record to another, but we weren’t concerned about that.”

And the band had no qualms about wanting to make it big. “I was trying to make up songs that I loved and to reach a big audience with them. There was a certain swinging for the fences thing about us. Back then everyone listened to college radio and everyone went to clubs, so we were kind of a big little band. There was a common scene, not like it is now where everyone has their little earbud niche.”

Like many Boston bands, they were doing fine until they signed to a major label. The one album for Epic Records, 1991’s “Toyboat Toyboat Toyboat” went over well with local fans — but that’s about all who ever heard it. “Right band, wrong time, wrong label,” Herlihy says. “Just as our album was about to come out, our product manager got fired and a new president came into the label. From then on we weren’t a priority for them.”

In fact he can remember commiserating with the singer of another, then-unsuccessful Epic band. “I remember talking with (Pearl Jam frontman) Eddie Vedder about how bad Epic was. We’d make ourselves feel better by going into offices and loading our bags with CDs. Then Nirvana hit and Epic said ‘We have a band from Seattle too, right?’ Otherwise they would have been flushed out with us.”

Herlihy played a few shows behind a solo album last year, but he also has a longstanding legal career and now teaches music industry at Northeastern. And he says that this show will likely be O Positive’s last hurrah. “Never say never, but I don’t see it happening again. I love to play with my friends but I can’t imagine pushing a career. If we had a band now trying to do what O Positive did, it would take ten people working full time.”