Add strings and multiply

The harp on "Sweet Jane" has to go.

At least that's what Cowboy Junkies guitarist Michael Timmins thought after hearing the demo of Sean O'Loughlin's arrangement of the tune, which the Canadian band will play with the Boston Pops tomorrow night and Sunday.

"To me, that sound or that feeling isn't going to work there," Timmins says, referring to the Lou Reed song, which the Cowboy Junkies recorded so memorably in 1988.

O'Loughlin, a New England Conservatory graduate, wasn't surprised. As an arranger with a special knack for bringing orchestras and rock bands together, he regularly deals with doubts on both sides. The rockers typically can't read music, thriving instead on groove and feel. The orchestral players can blow through the most complicated pieces at first sight, but they need sheet music. Enter O'Loughlin, 34, who moved to California shortly after earning his master's degree in composition at NEC in 1997.

"It's not a slam dunk when you bring in a rock band to play with an orchestra," he says. "It's one thing if you're a five-piece band. It's another if you're in a 90-piece orchestra. What's the line from 'Back to the Future'? 'B-flat blues, follow me and watch for changes.' That doesn't work."

In his other life, O'Loughlin is a composer and a conductor. What sets him apart, though, has nothing to do with his gift for writing or arranging soaring melodies, says Johanna Rees, a program manager at the Los Angeles Philharmonic. She hired O'Loughlin to arrange the music for a sold-out Belle & Sebastian show at the Hollywood Bowl last summer.

"Sean disarms people with his humility," says Rees. "He has no ego, and that's huge. This won't work with someone who says, 'I am an artiste, and how dare you touch my music.' "

O'Loughlin doesn't mind keeping his ego in check. He's got enough to worry about. "Pops on the Edge," the series launched in 2005, is meant to drive younger people into Symphony Hall by creating high-profile collaborations between the orchestra and rockers. But the same Pops rules of rehearsal apply. Bands get only a single practice with the orchestra, and O'Loughlin's work takes place under tight deadlines.

He starts by listening to what the band has in mind for an orchestration. Then, using his Mac and a music notation program, O'Loughlin works, as he describes it, like a painter, dropping different musical colors onto the page.

"What I try to do is create an orchestral score that matches how the band normally plays the score," says O'Loughlin. "I don't want you to go to a rock concert and say, 'Oh, an orchestra showed up.' "

Not every rock song works with an orchestra. But when the right choices are made, he says, the music is heightened, and made deeper.

"Do you think 'Live and Let Die' would have the same impact if the orchestra were left out?" he says. "Or 'Sgt. Pepper's Lonely Hearts Club Band?' I'm bringing this type of impact and historical context to the indie bands of today."

Though he's been working increasingly with rock bands, O'Loughlin's no Top 40 junkie. He says he likes Sting and Peter Gabriel, but he's a bigger fan of Stravinsky and Copland as well as film score masters like John Williams and Bernard Herrmann. O'Loughlin's taste does lean toward the melodic, as opposed to atonal, a fact that earned him a kind of outsider status at NEC, says a former classmate.

"He was interested in tuneful music in an environment that's typically very interested in 'artful music,' " says Chris Avery, a friend and former composition major at NEC. "The initial reaction [to his work] was to sort of pooh-pooh it, but in the end, a lot of people found it refreshing."

It was at NEC that O'Loughlin got his first big break. In 1995, he had taken a job as a bartender at Symphony Hall to earn extra cash, and hear the Boston Symphony Orchestra for free. His boss, John MacMinn, heard he had written a piece in honor of the 100th running of the Boston Marathon. MacMinn gave a copy of the piece to Dennis Alves, director of artistic planning for the Pops. Alves was impressed. In June 1996, the Pops performed the work at a concert.

"I was 23 years old," says O'Loughlin. "It gave me a lot of confidence to know I could do this. I used that confidence to move out to Los Angeles."

He first thought he would score films and was accepted into the University of Southern California's film program. But O'Loughlin quickly realized how difficult it was for an unknown to get work. He didn't want to take a second, nonmusic job to pay his rent. So he decided not to go to USC, instead moving into music publishing. O'Loughlin writes original music for school orchestras -- sorry, no marching band music -- and so far has published 90 compositions.

And his arranging took off in 1998, when Pink Martini, which blends dance music with jazz and pop, needed somebody good -- and cheap -- to help the group prepare for a gig with the Oregon Symphony . Rob Olivia, a former NEC librarian then working at the Oregon Symphony, suggested O'Loughlin.

"At that point, Pink Martini wasn't as big as today, and we were operating on a shoestring," says Olivia. "Sean worked up some arrangements very quickly and about a day later, we put them on stage, and they were terrific."

Since then, O'Loughlin has worked with Nickel Creek, Paris Combo, Blue Man Group, and, starting this summer, the Decemberists, who he will also conduct in four dates.

And, of course, there are the Cowboy Junkies.

"I've met him and talked to him and basically put my trust in him because, as I told him, I can listen to these things , but I'm not going to know until I sit with the orchestra and by then, it's too late," says Timmins.

That's not exactly true, says O'Loughlin, who will arrive in Boston today for time with Pops conductor Keith Lockhart and Timmins. Tomorrow afternoon, the Cowboy Junkies will have its lone rehearsal with the orchestra.

So for now, the harp stays in the mix.

"It just adds so much, it adds a richness," says O'Loughlin.

But what if Timmins doesn't agree? The arranger doesn't hesitate.

"We'll cut it out," he says.

Geoff Edgers can be reached at gedgers@globe.com. For more on the arts, go to boston.com/ae/ theater_arts/exhibitionist.