The members of O.A.R. could easily walk through Montgomery Mall — five miles south of Rockville’s Wootton High School, where they formed 15 years ago — without being chased, without being asked for a photo, maybe without even being noticed. They’re both popular and anonymous. Stealth rock stars.

Three hours before a recent Pittsburgh gig, they are gathered backstage for dinner, reminiscing about their days at Wootton, forks stabbing at iceberg lettuce and chunks of cheesecake. Like their music, they’re pleasant, sincere and mellow.

All along, their fans have been devout. Since self-releasing their 1997 debut, “The Wanderer,” while still in high school, O.A.R. has sold more than 1.8 million albums — a number that will grow when the quintet’s seventh album, “King,” drops on Tuesday. They’ve sold out Madison Square Garden. Twice. They’ll try to sell out Merriweather Post Pavilion on Aug. 13. It’ll be the grand finale of a 25-gig summer tour they’ve compared to a series of class reunions.

The comparison is apt. O.A.R. works hard to maintain an intimacy with its fans. The group hasn’t entered our greater cultural bloodstream — they’ve never graced the cover of Rolling Stone or performed on “Saturday Night Live.” That might be why their most fervent followers still see them more as friends than superstars.

“There’s never been a divide, almost to a fault,” says frontman Marc Roberge, who finds himself in constant touch with fans he barely knows. “Your family’s looking at you like, ‘Who are you texting right now?’ ‘Oh, ya know, it’s Stanley from Detroit.’ ”

The band members — Roberge, drummer Chris Culos, saxophonist/guitarist Jerry DePizzo, bassist Benj Gershman and guitarist Richard On — have been invited to fans’ weddings — and even attended one. At a recent concert, a kid on her mom’s shoulders chucked an envelope onstage. It was an invitation to her seventh-birthday party.

And while O.A.R.’s loyal following has ballooned over the years, the band has never really crossed into the mainstream — perhaps because there’s no longer a mainstream for rock bands to cross into. With the splintering of the music industry, rock stars (or the 21st-century semblance of them) are now made in the hippest circles of the blogosphere, a place where O.A.R.’s easy, breezy, populist rock has never stood a chance.

The industry, meanwhile, has struggled to define O.A.R., calling it frat rock, a jam band, a Dave Matthews knockoff — all titles that make these guys cringe. For years, they dubbed themselves “island vibe roots rock,” but it didn’t stick. Nowadays, the only label they embrace is “underdog.”

“I haven’t always felt like the industry understands what we are,” Gershman says. “The underdog spot . . . it’s always driven us to stay with it and keep redefining ourselves.”

The group’s visibility increased a smidge in 2008 when it signed with Atlantic Records and began to license music to car commercials and television networks. But the band has since left Atlantic for Wind-up Records, the label that once launched Creed to superstardom.

But O.A.R. is content with success on its own terms. Without any real peers to compare themselves to, they’ve turned the great unknown into a comfort zone.

“Sometimes you feel lonely,” Roberge says. “But it’s like this sweet loneliness because you don’t feel like you’re in the wrong place.”

The band is currently spread out all over the place, with members residing in New York City, Chicago, Arlington and Columbus, Ohio. All are 32 and married, except for Gershman, who’s 31 and single.

The name is short for “. . . Of a Revolution,” taken from a short story Roberge penned in high school. It was 1996, and every day after school in Culos’s basement, Roberge was also penning reggae-tinted rock songs that would resonate with his classmates — kids raised on a steady diet of Sublime, 311, No Doubt and the Dave Matthews Band.

On Thursday nights, Of a Revolution played the Grand Marquis Cafe in Olney, not just for fellow students who tried to sneak beers but for parents and teachers who turned blind eyes.

“Our angle was always this communal thing,” Roberge says. “You could go there and party and not worry about being in trouble the next day. It was a free pass.”

After recording “The Wanderer,” the band decided that it didn’t want its glory days to end on graduation day. They strategized a way to transplant themselves to Ohio State University in Columbus.

“We said, ‘Let’s go to the biggest college we could possibly get into,’ ” Roberge remembers. “We weren’t screwing around. We wanted to be in a band. We didn’t want to be famous — we wanted to be in a band together.” Their parents actually gave the green light.

“I thought it was an absolutely great idea,” says Carl Culos, Chris’s father. “My only fear was that it was all going to blow up and I was going to have four other sets of parents chasing me all over the neighborhood.”

Carl Culos — a drummer who once played with the likes of Edwin Starr and Stanley Mitchell and still works at Chuck Levin’s Washington Music Center in Wheaton — had been helping the band with its gear since Day One. Meanwhile, Marc’s father, Bill Roberge, gave the band business advice. In the fall of 2000, all of the band’s parents had a chance to see it if it had paid off. The band — now dubbed O.A.R. — had rented out Columbus’s Newport Music Hall for a gig that overlapped with parents weekend at the university.

“Lo and behold, it sold out,” Bill Roberge says. “It became clear that they were doing something that people really wanted to listen to.”

It was also clear that O.A.R. had become ­­savvy marketers. Before leaving Rockville for Columbus, they asked friends bound for big state schools to take boxes of CDs with them. The friends would peddle the CDs and the band would go play the campus.

Online, Napster was exploding and the group embraced file sharing as a way to promote its live shows. In 2000, the group’s fan base began to coalesce on message boards where fans would trade live recordings. Soon, O.A.R. were taking tour buses out of Ohio for the weekend and rolling back into campus on Monday morning.

“I got back to class like, ‘Pfffft, I’m outta here,’ ” On says.

Culos and Roberge finished their degrees, but the others, including DiPizzo, who joined the band once they arrived in Columbus, decided to hang it up early and put their full energy into O.A.R.

A lot of that energy was spent engaging their fans, interacting with them before, during and after each show.

“I think a lot of artists make the mistake of living in this bubble where they just don’t pay attention to what’s going on,” Roberge says. “We’re quite the opposite. We want to know what our fan base expects.”

They still read fan-operated message boards and post on them, too. In May, Roberge logged on to OARFans to explain why the release of “King” had been delayed — his wife’s cancer, diagnosed in September. “She is doing very well now,” he noted.

The band has also used that direct connection with fans to address their wild behavior. There’s a peculiar and sometimes ugly side to O.A.R.’s followers, a largely college-age crowd that likes to party hard. In 2004, a triumphant homecoming gig at Wolf Trap was spoiled when police cited 46 people for underage drinking. In 2008, 20 minors were taken to hospitals after the band performed at Jones Beach Theater outside New York.

This year, the group adopted a fan code of conduct to try to get the bad apples to settle down and behave — a tack they say has been effective. For a band that’s defined by its flock, they’re rightfully concerned.

“I’ve never once in 15 years onstage said, ‘Go get hammered,’ ” Roberge says. “However, we’re not dumb. We know that, for some reason, it’s become that kind of party atmosphere at times. We think that’s fine. We want it to be a party. We just want to send a message that we’re not encouraging you to go over that line.”

It’s happening less and less, but Roberge occasionally has to reprimand kicking crowd-surfers, overzealous mosh-pit rookies and unwelcome dudes grinding up on girls.

How O.A.R.’s relaxed, upbeat tunes spark such havoc still leaves the band perplexed.

“Sometimes when people feel really good about something, they put it out in a different way, I guess,” Roberge says. “That’s the only explanation I can think of.”

An hour later, Roberge is out by the tour bus, fussing over the evening’s set list like it’s a toast he’s about to deliver at a big party.

“The fans here are great,” he says. “I used to have this thing where I couldn’t call them ‘fans.’ It was always ‘the audience.’ ”

DiPizzo is backstage waiting on the set list, rehearsing the band’s new horn section through Outkast’s “SpottieOttieDopaliscious.”

Outside, fans are flip-flopping around the venue. In the concession line, a bespectacled beerista finds herself reciting a mantra to customers: “You have to finish that one before I can pour you another.” Chug, chug, chug.

When the band takes the stage, the crowd seems overjoyed. And relaxed. And gregarious. The only punches thrown are soft ones in the arm from young men who just want to give you a high five, bro.

A crowd surfer goes up during a new tune called “Dangerous Connection.” Like so many O.A.R. songs, it politely asks to crash on the futon in your head for a few days. Then it stays for a week and eats all of your cereal. You have to call in the Beatles or Stevie Wonder to evict it.

Fans don’t fight it. They roar for their old favorites — “I Feel Home” and “A Crazy Game of Poker”— but the new tunes go down just as smooth. On the band’s latest single, “Heaven,” Roberge sings, “Out my window there’s a million lights/A thousand hearts feeling just like me.”

There are no snowflakes at an O.A.R. show. This music feels like counter-programming to the narcissism that dominates contemporary pop music. It’s played by secret stars who have figured out how to make the middle of the road feel like counterculture.