It was late in the fourth round of the Othello Grand Prix Finals Saturday afternoon at the Capital Hilton. A small knot of spectators gathered around the table where Brian Rose was battling it out with George Sullivan. Rose slapped a disc down in a corner of the 64-square Othello board and turned over five of his opponent's pieces, changing their color from black to white. Without pausing to scan the board, Sullivan (who was in time-trouble as usual) placed a disc on the square next to Rose's corner and turned over one little chip. To alay observer, it looked bad for Sullivan, one of the game's leading theoreticians.

Brian Rose, a 17-year-old hotshot who goes to school at Hunter College High in New York, might be the Bobby Fischer of the ancient but born-again game of Othello. But that distinction is already nailed down by another teen-age gunslinger: Hidenori Maruoka of Japan, who captured the world championship when he was 17.

"Rose wasn't even playing in tournaments a year ago," whispered tournament director Mark Weinberg speaking softly to avoid disturbing the players' concentration. "He's a whiz with Rubik's Cube, too," said John Stoner. "Last night, I saw him do one in less than a minute. I don't even dare to own one of those things."

Suddenly, Rose was helpless -- making forced moves that strengthened his opponent. Sullivan would put down a tile and rows of pieces would flip from white to black. Rose counterpunched, snagging most of a diagonal line on the final move, but when the score was counted Sullivan had won -- though just barely, 31 to 33. "Well, we have a contest again," someone said. The unknown New Yorker had stumbled after going through his opponents like a buzz saw in the first three rounds.

An Othello tournament is unlike a chess tournament, first of all, because it is a lot smaller. Millions of copies of the board game have been sold by Gabriel Industries, a CBS subsidiary, since it was copyrighted in the mid-'70s. But most people play it at home with friends, unaware that there are national and international organizations, that strategic theories are being worked out and tested, that you can play Othello by mail and that a world championship tournament will be held in Brussels in October. "Our national office is Post Office Box 342 in Falls Church, Va.," says John Stoner, president of the U.S. Othello Association and inventor (or discoverer) of a ploy in the game called "Stoner's trap." "It's not very roomy, but we're a small organization."

The association has somewhat less than 300 members, who pay $5 a year for a magazine called Othello Quarterly, play in tournaments and destroy one another with a kind of friendly camaraderie rare in the more crowded, intense world of chess. "Othello players tend to be nicer people than chess players," says Mark Weinberg, who plays both games. Weinberg, a Washington tax attorney, was the catalyst in founding the organization. "The best players up and down the East Coast couldn't find any real competition in their own neighborhoods," he says, "so we had to get organized."

Nobody knows how old Othello is or where it was first played. "It keeps being discovered and forgotten," according to George Sullivan, a securities analyst and free-lance writer from Alexandria, who first learned about the game when he was asked to co-author an article on it for Games magazine. "It was played centuries ago in England under the name of Reversi, and Martin Gardner wrote a couple of columns on it in Scientific American in the early '60s, but it didn't take off then because nobody saw anything in it that they could copyright."

What Gabriel finally copyrighted was the name "Othello", and began marketing the game with the slogan "a minute to learn . . . a lifetime to master." The game is played with 64 checkers, black on one side and white on the other. It begins with two pieces of each color already set down in the center of the board. On each move, a player must put down one disc and flank at least one piece of the opposite color -- horizontally, vertically, diagonally or all three. Pieces that are thus flanked are turned over, changing their color. This constant change, sometimes involving a dozen pieces or more, leads to quick, dramatic reversals in the situation. The player who has the most pieces of his color at the end is the winner.

That's all there is to it -- or all there has been through the centuries before Reversi became Othello and began to be intensively marketed. It was like chess in the first thousand years of its existence -- a pastime notable for random cleverness but with no organized theory. Now, in Othello Quarterly, theoreticians are discussing the value of corner positions, wing variations in the opening, perimeter discs, and such exotic ideas as "evaporation technique." Computers have been programmed with the game, including one on the way from Gabriel Industries and another from Fidelity Electronics, whose home computers already play chess, bridge and backgammon. Fidelity has to call its product "Reversi Challenger," but the game it plays is a recognizable twin of Othello.

Computers and humans alike, intensively working on the deceptively simple game in the past few years, keep finding new strategic depths. "Nobody knows how deep the game is," says Stoner, "because the horizon keeps receding before us. We seem to reach a plateau, then someone will break through, publish his discovery, and we all go up a notch."

"So far, we are keeping tournament games short," adds Stoner. "Only 25 minutes per player. That way, we can hold a seven-round tournament in one day."

"That may be one of our problems in trying to get Othello established as a serious game," says Sullivan. "Chess players think that any game that's over in an hour can't be very deep."

Anyone who watched the eight players in the Grand Prix Finals sweating to choose the right move would have no trouble believing in the game's depth. And anyone who concentrated on Brian Rose, struggling back from his fourth-round defeat with three straight victories in the final rounds, would agree that the 17-year-old was looking far into those depths and earning his first-place trophy.

Rose's second major challenge came in round six, when he faced David Toth of Freehold, N.J., the highest-rated player in the tournament and a runaway favorite to win it. Toth had already lost to Sullivan -- a heartbreaker with a two-point margin -- and then he dropped another game in round five. His encounter with Rose gave him one last chance to tie the score, and since Sullivan lost his game in round six it could have ended in a three-way tie. But the 17-year-old, by now recovered from his loss to Sullivan, played with the coolness of a pro, looking totally unconcerned as he put down his discs and maneuvered his opponent into a 16-point defeat. "You have to call it grace under pressure," said an onlooker. Rose's victory will seed him into the finals of the U.S. Championship in New York in October. The winner of that tournament will be sent to the world championship in Brussels, with Gabriel paying his expenses.

After the prizes were awarded, the U.S. Othello Association held a very brief and informal annual meeting, highlighted by treasurer Sullivan's report that "we have plenty of money." Then winners, losers and fans alike all went out for pizza. "Going out for pizza is a tradition," said Weinberg, "just as holding the Grand Prix in Washington and having me for tournament director are traditions. These traditions are only two years old, because we haven't been around long enough for older traditions. But we're working on it."