The Offspring get so much airplay in the nation's capital, they might as well be from here instead of California's increasingly famous Orange County. At WHFS, it sometimes sounds like around-the-clock Offspring -- perhaps the reason the band is co-headlining Saturday's day-long HFStival at the Robert F. Kennedy Memorial Stadium with the Cure. Others on the bill include Jay-Z, Papa Roach, O.A.R., P.O.D., Yeah Yeah Yeahs, Yellowcard, Violent Femmes, Cypress Hill, Modest Mouse and New Found Glory.

Of course, it helps that such melodic pop/punk offspring as "Come Out and Play (Keep 'Em Separated)," "Self-Esteem," "Pretty Fly (for a White Guy)," "Why Don't You Get a Job?," "The Kids Aren't Alright" and "She's Got Issues" have become modern-rock staples around the country.

Even so, lead singer and chief songwriter Dexter Holland says "it's weird how things pop up."

Like hearing Offspring songs performed by the world-famous University of Southern California Marching Band.

"I was dozing on the couch at home one day and a USC football game was on. And as it was going to commercial I heard something familiar, and I thought, 'Wait a minute, that's my song, it's "The Kids Aren't Alright!" ' I actually went down to USC and said [to the marching band], 'Hi, we should be friends.'

"And now they've got five of our songs in their repertoire."

Including the Offspring's most recent radio standard, "Hit That," unveiled during the eventual national co-champion's 47-22 drubbing of crosstown rival UCLA in November.

And don't think that wasn't fun for USC grad Holland, who received his bachelor's degree in biology in 1988 and a master's in molecular biology in 1990.

"I was in the middle of my dissertation for a doctorate in molecular biology, so I'm sure I would have finished that degree and that probably would have had me teaching at a university or something."

Except that in 1994, the Offspring, who had been laboring in genial obscurity since forming in high school in 1984, released an album called "Smash," which lived up to its name, selling 4 million copies in the United States and 7 million more overseas (the biggest-selling indie rock album ever) and, along with Green Day's "Dookie," ushered the pop-punk revival into the mainstream.

That doctorate? It's on hold, though Holland would like to nail it down eventually, if only to keep up with his punk brethren.

"Milo [Aukerman] from the Descendents has a doctorate in microbiology, and Greg Graffin from Bad Religion has one [in evolutionary biology-paleontology]," Holland says, adding with a chuckle that "with punk bands, the secret life is doctorates instead of drugs and animal cruelty!"

Officially, this is the Offspring's 20th anniversary, which leaves the 37-year-old Holland "shocked. It's terrible when you say it like that! We're not that old, though maybe we're old in MTV time . . .

"But for the first 10 years we were around, no one knew who we were," he says. "We were just doing it for fun in our garage. No one knew who we were until 'Smash' came out, so in a way it's more like 10 years. But, technically, we started all that time ago."

Back when O.C. was just O.C., not The O.C.

"For a long time people didn't know what Orange County was," Holland muses. "It was just part of L.A., didn't have its own identity. Now it's got a TV show. And it's called 'The O.C.' I don't know where the 'the' comes from. It's just Orange County, not The Orange County! Maybe that makes it sound better. But it's funny, as a TV portrayal."

As Holland told The Post a few years ago, O.C. punk was a reaction to the complacent Norman Rockwell lifestyle of the wealthy and conservative region, "a way for kids to rebel against the boredom and the feeling that they must fit a certain mold. . . . It looks like 'Happy Days' on the outside and feels like 'Twin Peaks' on the inside."

Holland and the other original band members, bassist Greg Kriesel and drummer Ron Welty, were fellow high school students who got together after they were unable to get into a Social Distortion concert; guitarist Kevin "Noodles" Wasserman, a custodian at their school, signed up a year later. The Offspring released a self-produced album in 1989 and then signed with Epitaph, with 1993's "Ignition" setting off underground buzz that went mainstream with "Smash."

More than 30 million albums later, the Offspring's canny meld of punk aggression and pop craft has made them and No Doubt the most successful bands to emerge from O.C., and both tap into the county's long-term love affair with ska. The band's latest album, the three-years-in-the-making "Splinter," features "The Worst Hangover Ever" and some of the least-convincing repentant lyrics this side of the country. There are social critiques ("Neocon," "Hit That," "The Noose"), romantic blues ("Spare Me the Details"), introspective tracks ("Race Against Myself," "Never Gonna Find Me") and roughhouse rockers ("Lightning Rod" and "Da Hui," a tribute to hard-core Hawaiian surfers).

The album's weirdest offering is the closing "When You're in Prison," in which Holland channels Al Jolson via some muffled crooning and the crackle of a well-worn 78; with a "don't reach for the soap" punch line that's even more archaic than the sound.

According to Holland, "It kind of popped into my head one day about five years ago. I hummed it into a little tape recorder so I wouldn't forget it and put it to the side, thinking, 'What am I ever going to do with that?' The more I started thinking about it, the more I decided I should just record it because it would be fun. That's part of it, too. When you've done a bunch of albums, you want to challenge yourself to do something a little different, and the idea was, can we make this sound like a '30s song with some lyrics that were really inappropriate just to balance it out."

Fans needn't worry about a major change in the Offspring sound: Those crunching melodies and power hooks remain intact. The only real change is in the drum chair, where Rocket From the Crypt alum Atom Willard now does the skin bashing. Ron Welty left a couple of years ago and Josh Freese (the Vandals, A Perfect Circle) handled the job during the recording of "Splinter" before Willard signed up.

"It has worked out great," Holland says. "It was strange at first because we didn't know Atom personally before. We auditioned about 10 guys, and he was just the best guy. It's important to have someone who can play great but also someone that you can hang out with on the bus, and it's been great because he's a really easy guy to get along with. And having been with Rocket From the Crypt, he'd done the whole tour thing and video shoots. I feel like we're sounding better than ever and getting along better than ever, too."

Sadly, it's Welty, not Willard, who is represented in the Offspring Playset, featuring three-inch-tall articulated action figures of the Offspring, including DJ Mark Moreno, and a touring stage. That's probably aimed at the band's younger fan demographic, though the Offspring have managed to appeal to a wide-ranging audience by not pretending they're never going to grow up.

"Hey, I'm a guy in my thirties and it's hard to write about hanging out in the quad, for sure," Holland says. "We've always tried to address issues that are maybe a little more personal and introspective, or maybe a little more social, like 'The Kids Aren't Alright.' Those kinds of subjects do appeal to young people primarily, but they're the kind of things that me and people in their thirties and forties are reexamining later on, like who they are and what their lives are like. So we see a wide age range at the shows. We're having fun doing this, and as long as it stays fun, we'll keep at it for a while."

But there are fallbacks, Holland says. In 1994, flush with "Smash" capital, he started the Nitro label, signing up O.C. punk originals such as TSOL and the Adolescents and offering a home to such new acts as Ensign, Rufio, A Wilhelm Scream and the Start.

"I always thought it would be cool to have a label like Epitaph and DisChord in D.C. And for some people that get 100 percent caught up in their band, it can be rough because it doesn't last forever . . . I like the idea of having other things going on."

And there's always that doctorate. Should Holland need to go back to school, he can pay his own tuition now. No more student loans.

"That's a whole lot less stress right there," he laughs.

OFFSPRING -- Appearing Saturday at the HFStival. * To hear a free Sound Bite from the Offspring, call Post-Haste at 301-313-2200 and press 8121. (Prince William residents, call 703-690-4110.)

The Offspring officially formed 20 years ago, but their 1994 album, "Smash," propelled their pop-punk sound into the mainstream.