THE HILARIOUS and frightening exhibition "Yesterday's Tomorrows: Past Visions of the American Fu- ture," at the National Museum of American History, has a local contributor. Michael O'Harro, better known as the owner of Tramps disco and now the Georgetown sports bar Champions, is a closet sci-fi buff -- literally. He has amassed one of the country's biggest collections of Buster Crabbe memorabilia, which he stashes unceremoniously in his closet, and nine of his Buck Rogers and Flash Gordon pieces are on loan to the Smithsonian for three years.

The late Crabbe, who was known as "king of the serials," was the only person to play both Buck Rogers and Flash Gordon in his multitudinous media incarnations, from movies to television. O'Harro says he became interested in Crabbe because of his dual role as Olympic champion and actor.

O'Harro says he's been collecting sci-fi stuff for three years, and had to put aside his prodigious baseball card collection (also one of the country's largest) for the Crabbe fixation. He estimates he spends several hours a day searching for the artifacts, and has paid as much as $1,000 for an item. Among his prizes are a Buck Rogers pocket watch (original price, 98 cents; estimated value today, $1,000) and a copy of the Amazing Stories comic book in which Buck Rogers made his debut.

O'Harro also has one of the country's largest ray gun collections. In fact, he just returned from California, where he bought what he considers to be the new jewel of his collection: the original papier-ma che' prototype of Buck Rogers' ray gun. "Yesterday's Tomorrows" is on view through Sept. 30, when it leaves for a national tour.