TO his students, Tim O'Brien is the guy with the lantern jaw who will go to the gym nightly, if he can, and teach boxing moves without charge. To the publishing world, he's the man who knows President Clinton better than anybody: every stray hair, every pore.
Mr. O'Brien, who lives in Park Slope, has treated magazine readers to his hyperrealistic oil-painted illustrations for five years; depictions like a vastly enlarged head of Bill Gates moored to the ground by a Lilliputian Janet Reno. But he has shared his love of boxing with the community for even longer, before he ever created his first Time cover.

Darryl Hayes, 31, began training with Mr. O'Brien nearly three years ago, when he often noticed a man who looked like the actor Rob Morrow, and was proudly Irish, judging by the shamrock tattooed on his right shoulder, training gangly teen-agers, professional women and other unlikely bruisers at the Y.M.C.A. on Ninth Street in Park Slope, where Mr. Hayes played basketball.
''I saw him teach people who had never known anything about the sport and take their skills to a new level,'' Mr. Hayes said. ''He was so patient. It never bothered him to repeat things over and over.''
Mr. O'Brien, 33, approached Mr. Hayes one night and asked him if he'd like to learn to box. The lessons, he added, would be free. The only prerequisites were the protective cloth wraps for hands and a willingness to learn. Mr. O'Brien would provide gloves if needed. Mr. Hayes trains three nights a week with Mr. O'Brien and other 'Y' members, who joined the free sessions by watching and getting an invitation.
''I love the sport and I'm good at teaching,'' Mr. O'Brien said.
Some students come to learn the basics: stance, jab, hook. Others are skilled boxers and, at Mr. O'Brien's suggestion, have tested their prowess at Gleason's Gym in Brooklyn Heights.
Mr. O'Brien's employers find it hard to believe that he likes to go a few rounds as a release from a day of painting. He is thought of as too easygoing and affable to excel in such an aggressive sport. But there are also practical considerations.
''When I found out that Tim liked to box, the first question I asked him was, 'What about your hands?' '' said Ken Smith, assistant art director at Time and a frequent commissioner of Mr. O'Brien's work. ''But he assured me, 'I'm the best wrapper in the business.' ''
Mr. Smith is more familiar with Mr. O'Brien's ability to paint images of newsmakers. Typically, he assigns an illustration to Mr. O'Brien on a Wednesday afternoon. By deadline on Friday, a painstakingly detailed painting is delivered. Among the other illustrators hired by Time, Mr. Smith puts Mr. O'Brien on his ''short A-list.''
''Tim is at the top of his game now,'' agreed Terrence Brown, director of the Society of Illustrators, where Mr. O'Brien is a board member. Art directors see Mr. O'Brien as one of the first artists they will call for a top-quality illustration of the President, Mr. Brown said.
To endure the rigors of an art assignment for Time, Mr. O'Brien said, he fuels up on energy food and takes short crash naps. He sees the process as similar to preparing for a boxing match. ''Both are physical, solitary endeavors,'' he said.
Mr. O'Brien entered boxing through his artwork. The middle son in a family of three boys, Mr. O'Brien loved to draw and was not athletic like his brothers. But at 9, when his father died, he took a sudden interest in portraying Muhammad Ali.
''Not having a father anymore, I had masculinity issues,'' Mr. O'Brien said. ''I wanted to be tougher. Ali was remarkable, handsome and always attracted a crowd. I picked him as my father.''
Mr. O'Brien's amateur boxing career began in his hometown, New Haven, with a mailbag he had stolen, stuffed with clothes and used as a punching bag. At 13, he took boxing lessons at the Police Athletic League in Wallingford, Conn., where he developed the agility of a solid fighter. He stopped training to study illustration at Paier College of Art in Hamden, but after college he moved to Philadelphia, worked as an illustrator and resumed boxing at a gym. The staff respected his boxing skill and rapport with inner-city children. Soon, he was asked to teach boxing to some of them. They called him Rocky.
''I was the only white guy, I had a crooked nose and it was Philly,'' Mr. O'Brien said.
He moved to Brooklyn in 1991 to be with his future wife, Elizabeth Parisi, who was his art director at Scholastic Inc., the educational publisher. (They met at a business lunch, after Ms. Parisi had become an avid fan of Mr. O'Brien's highly intricate children's book covers.)
Since the move, most of his students have been adults. He says he'll teach children, but with hesitation. In Philadelphia he agreed to let two young brothers live with him temporarily after he saw the boys' family being evicted from their home. Three months later, he realized that the boys' mother had abandoned them, and he returned them to their grandmother.
''It was defeating and upsetting for Tim,'' Ms. Parisi said. ''There is a fear that he will get attached like that again.'' He still keeps in touch with one brother.
Mr. O'Brien makes it clear that he doesn't teach street fighting. In a recent sparring session with two advanced students, he stopped the bout when one boxer's punches began to resemble falling anvils.
''He was getting emotional,'' Mr. O'Brien said. ''Boxing is not about emotion. It's a smart sport.''
Women, who spar with both sexes, are especially capable of learning the sport because they tend to have fewer preconceptions of how to box and are more focused on technique, Mr. O'Brien said. Kathy Grabarczyk, 37, began training with Mr. O'Brien after knee surgery made it difficult to continue her aerobic routine. After eight months, Ms. Grabarczyk became Mr. O'Brien's sparring partner, an honor that required her own boxing gear.
''I couldn't see myself at Gleason's before,'' said Ms. Grabarczyk, who competed in her first amateur event at the predominantly male gym last fall. ''But Tim makes you feel like you're a real boxer.''
There is rarely any crossover between Mr. O'Brien's career and coaching. But in 1997, Mr. Hayes was given the opportunity to repay Mr. O'Brien for years of dedicated training by posing as an injured basketball player for one of Mr. O'Brien's assignments for Sports Illustrated. Occasionally an illustration will break the warm-up routine and inspire discussion.
During a boxing session, on a recent Monday, several of Mr. O'Brien's students commented on their coach's latest work, a rendering of Linda Tripp in Time that was eerily lifelike, except that her face was transformed into a tape recorder.
''I got a lot of mail on that one,'' Mr. O'Brien said. ''Someone even suggested I was evil. But I spent a lot of time trying to make her hair look pretty. It goes both ways.''
He finished wrapping his hands and began to pummel a speed bag.

Published: 11 - 12 - 2000 , Late Edition - Final , Section 14 , Column 1 , Page 13