When Susan O'Brien, now 21, began swimming at Wildwood Manor as a youngster, driving across Montgomery County for a Saturday morning meet was considered a major trip. Now she does her competing in Sweden, Germany and Yugoslavia.

Recently, O'Brien was in Zagreb, Yugoslavia, as part of the U.S. contingent at the World University Games, an Olympics-style athletic event for the world's college students. She won a silver medal in the 100-meter backstroke, finishing in 1:03.77 and was part of the United States gold medal-winning 400-meter medley relay team.

O'Brien was chosen by U.S. Swimming, the sport's governing body in this country, to compete in the 100-meter backstroke, based on her performances in national championships and in the NCAA meet, where she finished fourth.

O'Brien tasted success early in her swimming career, attending her first national championships in 1979 when she was 12. She finished 22nd in the 100-meter backstroke in East Los Angeles that spring. Five years later, the summer before she left for the University of North Carolina, she won that event in Fort Lauderdale, Fla., beating Olympic silver medalist Betsy Mitchell.

Going away to school and maintaining a high level of competitiveness has forced her to mature, O'Brien said recently, prior to flying to Yugoslavia. She has reached the stage of athletic development where she knows what she needs to do and what she is capable of.

O'Brien no longer speaks in terms of what her coach wants, but what she wants. Although she concedes twice-daily workouts aren't easy, O'Brien said her swimming is less demanding than her schoolwork.

Less than two weeks before leaving for Yugoslavia, O'Brien was in Chapel Hill taking summer school courses. In January, both aspects of her life were put to the test when her father died after a prolonged illness.

"This year was hard," she said, quietly. "School is very important to me even though I have a hard time with it. I've done well at UNC and swimming has helped me. It's hard when I'm not doing well to stay motivated.

"I don't love the training every day. I don't really enjoy that, but I think school, to any swimmer or athlete, is very important. Without swimming, I wouldn't be able to get in this school and, if I quit swimming, I wouldn't be able to pay for my education."

O'Brien relied on her swimming to pull her through the tragedy of her father's death, and she swam what she considered her best NCAA meet this spring. She came the closest to eclipsing the time of 1:03.05 she swam three years ago to beat Mitchell.

Her goal for the summer, as it is for the other top swimmers in the country, is to qualify for either the Pan Am or the Pan Pacific teams. Those teams will be chosen this week from the first four finishers at the national championships in Clovis, Calif.

Beyond that, O'Brien has her 1987-88 schedule mapped out. "Next year is my senior year and hopefully, I'll have my best NCAAs, then I'll train for the Olympic Trials," she said. "That or the Olympics will be my last meet, but the Olympic Trials are the farthest thing from my mind right now. When the NCAAs are over next year, I'll have to make a full commitment to swimming. There'll be no more college swimming. And the only reason I'll do it is for myself and to make the team. If I'm not doing it honestly, it wouldn't be worth it."