IT IS 7:30 on a Saturday night, and the Ontario Theater is embarking on a marathon of the three "Omen" movies, one after the other. Thirty people are rattling around in the great dark chamber which has room for 1,100. The floor isn't canted, to speak of, but the huge screen is so high that it doesn't matter. A stage projects several yards in front of it. The place is clean, amazingly clean.

"We have three films every night," says Seth Hurwitz, the former manager who now books pictures for the Ontario when he isn't running his own booking company, IMP ("(It's May Party"). "Three movies for $3, it's a gimmick. I go to all the screenings of first-run pictures and only use proven hits. The neighborhood is changing, and we try to keep that in mind."

The neighborhood is perhaps Washington's most interesting, Adams-Morgan, the Columbia Road area between 16th and Connecticut. Blacks, whites, Latinos, artists, embassy people, white-haired apartment dwellers . . . and he's right, it is changing, and the prices are going up.

"I tried 'Elephant Man,' and that didn't go. I tried 'Straw Dogs,' which has plenty of violence, but it didn't go. I put the classic 'Freaks' in with 'The Fantastic Animation Festival,' but that didn't work either. But they loved 'Gloria,' which is a classy movie but violent."

Now the audience is building. People drift in steadily, paying no attention to the movie times. A group, laden with cups, pails and bushel baskets of popcorn, files in and settles itself. On the screen, David Warner is being nastily beheaded by sheet glass, the sound track is screaming and blood is pumping, but the talkative newcomers don't bother to look.

Recently a local magazine attacked the Ontario for running so much violence, notably the sadistic "I Spit on Your Grave." Hurwitz and the present manager, Carlos Rosario, say they are doing their best to upgrade the product while still making a living. On weekends the theater shows Spanish-language films, mostly Mexican, with the occasional Cantinflas comedy (no subtitles). These do very well indeed.The live rock concerts also do well.

"It's expensive to operate as a concert Hall," Hurwitz says, "because there are no lights or sound, everything has to be brought in and taken out. We pick them carefully, charge $8 or $9, you have to be sure you have a hit. We had three this year, all sellouts."

One problem is making the theater attractive to suburban kids who might feel threatened by the neighborhood and perhaps don't understand the uninhibited Columbia Road audiences. The Ontario goes out of its way to have police protection at concerts and a couple of black-belt bouncers hanging around . . . "They're kids themselves, and they're concerned mainly with the fire regulations. It's a happy group, a little noisy, but we've never had any trouble," Hurwitz says.

The first "Omen" picture is over, and more people drift in from the black-marble-and-mirror lobby designed by Marvin Goldman when K-B Theaters took over the place in 1958. Someone calls to a friend clear across the theater. the friend shouts happily back. Small children run up and down the aisles. Everyone seems immune to the film's determined spookiness. It is only when the action explodes that the chatter stops, like crickets in the country when a car passes.

"Outrageous is our byword," Hurwitz says. "We don't do any X-rated stuff. I would say the ideal combination was 'Dawn of the Dead' and 'halloween II.' The perfect Ontario movie. Sometimes they come in late and don't like it and demand their money back, or they want to pay $1 just to see the last picture."

For several years he tried to run hard-ticket reserved-seat programs at the Ontario, but it was no good. The turning point came in March 1979, when Bruce Lee's "Enter the Dragon" opened, and receipts went through the roof. Since then, the Ontario Theater has provided a fascinating study in esthetics, teetering delicately on the razor's edge between art and money. It's also the last word in community movie theaters.

The other night they had a ridiculous picture called "Dracula's Dog." A guy came up to the box office and said, "I didnt know dracula had no dawg." But he paid his $3 and went in anyway.