Sean O'Neill does not play Ping-Pong. That is far too congenial a name for a sport where the ball travels over 100 miles per hour or rotates up to 141 times per second.

What Sean O'Neill does play is table tennis. And he plays it very well.

O'Neill, 18, of McLean, won the singles and mixed doubles national titles last month in Las Vegas and has found himself in demand ever since. A steady stream of national and local television crews, writers and photographers trudge through his home daily, and there is talk of a story in Sports Illustrated and television appearances with Johnny Carson and David Letterman.

In fact, O'Neill has become so familiar with the press that he hands out biographies he compiled on the computer in his bedroom and he plays table tennis videotapes on his wide- screen television to better illustrate points.

But all this attention has got him on edge. It's been three weeks since O'Neill played table tennis competitively, and he can sense the deterioration of his skills.

"My game goes right down the tubes if I don't train," says O'Neill. "A two-day layoff wouldn't hurt me, but a week can cause all of my shots to change."

Although he at first appears gawky, it's apparent once O'Neill settles in behind a table that he is a world-class athlete. Daily, four- hour practice sessions, which include hour- long rallies, combined with an added two hours of running, swimming, biking and weight-lifting, have left O'Neill equal to the test of a very strenuous and physical sport.

Unlike the mild, century-old parlor game of Ping-Pong (a name Parker Brothers patented in 1901), table tennis requires lightning reflexes and concentrated, but reserved, strength. In international competitions, players often lunge all over a space the size of a tennis court in an effort to keep the ball, weighing one-tenth of an ounce, on a table 9 feet by 5 feet. Torn muscles are not uncommon.

But table tennis has its rewards. Each winter, O'Neill takes a four-month training tour of Western Europe. He has a corporate sponsor (Stiga of Sweden), which underwrites most of his training and travel. And last year O'Neill earned approximately $5,000 in prize money, including one payday of $1,800.

"I'm really not in this sport for the money," O'Neill grinned, "but that's not too bad for one day's work."

O'Neill learned the game from his father, a pool-room whiz as a youth in Ohio, and began competing in 1976 at the age of eight. Since then he's sharpened his skills within the small, but dedicated, network of table tennis clubs in this area, and won tournaments in such diverse places as China, Cuba and Sweden. O'Neill is expected to anchor the U.S. squad that will compete in the 1987 Pan-American games and the 1988 Olympics.

"I can't count on that, though," says O'Neill. "There's always somebody challenging you, and any one of the top 30 players can beat the national champion on a given day. It wouldn't even be considered an upset."

But O'Neill's dedication to the sport is such that he has even begun to question his decision to attend George Washington University as a computer science major. (The school has even let him set up a practice table in its gymnasium.)

"I do intend to prepare for life through college," he says, "but I have no need for an education if it ruins my chances of an Olympic medal or a world championship."

So O'Neill is taking the spring semester off in order to train in Sweden, as he has for the past seven years. Although the U.S. is ranked 14th in the world, its top-level competitors are spread out across the country. By training in Europe, O'Neill faces a much more concentrated opposition.

But his absence will not leave the area bereft of table tennis talent. In fact, O'Neill's top local threat is his housemate and coach, Chartchai Teekaveerakit, the No. 1-ranked player in Thailand. The pair have come to so totally dominate area tournaments that they now vary their appearances to give other competitors a chance to reach the finals.

"Both of us have more ability now that we play each other regularly," says Chartchai, 18, who won the $1,000 first prize last year at the Duneland All-American Pro-Am in Indiana. O'Neill finished second.

"I could never have gotten this good if I had stayed in Thailand," Chartchai says. "If you want to be good, you have to practice every day like we do. You can't let up even for one day."

While Europeans are known for their defensive play and the Chinese for their attacking style, O'Neill says a truly American technique has yet to emerge.

"What you see is a conglomeration of styles," he says. "I guess it's sort of reflective of this country. We just kind of adopt the best everybody else has to offer." WATCHING TABLE TENNIS

The Howard County Open, which will feature top area players such as Qung Do of Gaithersburg, David Sakai of Lanham, 13-year old Jeff Harris of Baltimore, and, perhaps, Chartchai, will be held this Sunday from 9 a.m. to 8 p.m. at the Wilde Lake Middle School, 10481 Cross Fox Lane, Columbia. Admission is free.

Similar tournaments will be held at the school on February 9, March 2, April 12-13 and May 10-11. (To get there from the Beltway, take Route 29 north to Columbia. Turn left on Little Patuxent Parkway, right on Governor Warfield Parkway and then left on Twin Rivers Road. Turn left on Cross Fox Lane, and the school is the first driveway on the left.)

In addition, the Junior Olympic trials for the Middle Atlantic states will be held May 17 at the Loch Raven Middle School in Timonium. PLAYING TABLE TENNIS

The table tennis clubs in this area are informal gathering spots where experts can hone their skills, beginners can learn the pen grip, and fans can watch some of the best players in the country work out. The clubs are open to everyone. Some require fees for use of the facility and tables. Racquets and balls are not provided.

NORTHERN VIRGINIA TABLE TENNIS CLUB -- Meets Friday from 8:45 to 10:15 p.m. at Springhill Elementary School, 8201 Lewinsville Rd., McLean, (448-7733). Fee: $5 per month.

BALTIMORE TABLE TENNIS CLUB -- Meets Monday and Thursday from 7:30 to 10:30 p.m. and Saturday from 9 a.m. to 12:30 p.m. at Old Court Junior High School, 4627 Old Court Road, Baltimore, (301/255-6998). Fee: $25 per year.

HOWARD COUNTY TABLE TENNIS CLUB -- Meets Monday, Wednesday and Friday from 7 to 10 p.m. at Wilde Lake Middle School, 10481 Cross Fox Lane, Columbia (301/730-5626). Fee: $30 per year.

LOCH RAVEN TABLE TENNIS CLUB -- Meets Wednesday from 7 to 10:30 p.m. at Loch Raven Middle School, 8101 LaSalle Road, Timonium (301/252- 0740). No fee required.

METRO TABLE TENNIS CLUB -- Meets Monday and Thursday from 6:30 to 10 p.m. and Sunday from 3 to 6 p.m. at University of Maryland's Physical Activity Center, Room 108 (301/490-6805). No fee required.

TABLE TENNIS CLUB OF ROCKVILLE -- Meets Monday and Wednesday from 7:30 to 10:30 p.m. at Richard Montgomery High School, 250 Richard Montgomery Avenue, Rockville (670-9091). Fee: $60 per year, or $2 per night.

If you'd like to sharpen your skills in your own basement before presenting them to the world, table tennis is a relatively inexpensive sport. Tables, which cost anywhere from $70 to $100, are available at most sporting goods stores in the area, as are racquets ($10-$30) and balls ($2-$3).