The OAS building was taken over by Marxists last night.

Fans of the Marx Brothers gathered to honor Chico, Harpo, Groucho and even Zeppo and Gummo. Harpo's widow, Susan, was on hand. So were his son, Bill, Chico's daughter, Maxine Marx, and her son, Brian Culhane.

Groucho was represented by Charlotte Chandler, the author of "Hello, I Must Be Going: Groucho and His Friends." Chandler and OAS Secretary General Alejandro Orfila are friends, and the party technically honored her and, inexplicably, the International Year of the Child, as well as the Marxes.

But there was no question that Marx mania was the party's chief drawing card.

There was consideration speculation about how the Marx Brothers themselves would have behaved last night. It was a black-tie affair, with candles on the tables and a fancy buffet. The consensus was that the brothers would have made the ornate hall a shambles.

"Their whole attitude about anything the least bit staid and establishment was to destroy it," said Maxine Marx, a TV commercial cassting director. "Harpo and Daddy would have been sliding across the floor on their back sides, and Groucho would have been making remarks that were not at all flattering."

Harpo wouldn't even have come, said his widow, Susan, who is a civic official and former actress herself.

"He was a very simple, private person. We don't do things like this in California."

Even Orfila agreed that the brothers probably would have made a mess of his party. What would he have done? "Join them in laughing," he said.

Divisions in the family became apparent in the exchanges of the clan. Maxine said Harpo and Chico were "adorable," but "Groucho I'm a little ambivalent about. He could be so mean."

Maxine declared that Harpo and Susan "had one of the great happy marriages ever seen in that town (Hollywood)."

"On, that's good," replied Susan. "But then, how many people could have married Harpo?" On the other hand, she added, "it could have been Chico."

"You and Chico never saw eye to eye, did you?" asked Maxine.

"We never saw eye to eye on anything," responded Susan. She told several stories about Chico's habit of gambling or giving away his money, and Maxine nodded.

"Once someone asked Daddy how much money he lost," said Maxine, "and he said '2,550,632.48.' How do you know?' said the guy. So Daddy said 'Because that's how much money Harpo has in the bank.' That's why I call myself a member of the Nouveau Poor."

Paul Tendler of Washington broke in to tell Susan that he had wanted to name his daughter Harpo but had to compromise by using the name for his dog. Susan seemed more surprised that someone would think of naming a daughter after her husband than she did at the thought that someone would name a dog after him.

"I'd like to follow you people around," she said.

Maxine remarked that Groucho named a dog Chico "so he could go around saying 'Here Chico, here Chico.'"

The party was preceded by a screening of vintage Marx clips at the American Film Institute, including a recently discovered promotional film for "Monkey Business" that features a scene in which the brothers performed hilariously inept impersonations of Maurice Chevalier. The Chevalier bit was later incorporated, in slightly less hilarious form, into "Monkey Business" itself.

Chandler and the late director of the Cinematheque Francaise, Henri Langlois, found the promotional film in a private California collection. After Langlois died, the collector gave the film to Chandler, who is donating copies to the AFI, the Museum of Modern Art, the Lincoln Center Film Society and the Cinematheque, she said.

As guests left the party, they were greeted by approximately 30 candle-carrying black-masked demonstrators protesting what they termed "infanticide" in Nicaragua, a member of the OAS.

"In the Year of the Child," chanted the demonstrators, "children are dying." Some of the guests accepted leaflets from the demonstrators. CAPTION: Picture, Brian Culhane, Maxine Marx, Susan Marx, Bill Marx and Charlotte Chandler; by Linda Wheeler - The Washington Post.