You could find Richard Allen, the president's national security adviser, grazing on oysters at a party last night for South Korean President Chun Doo Hwan. Pretty soon, White House deputy counselor Robert Garrick wandered by. He's just been to California.

"Pasadena was delightful," Garrick told Allen. "The temperature was 72. I leased my house, and I went to church twice."

"Well, good," responded Allen brightly, "I did a communique."

"Boy, you really know how to go after those things," Garrick said.

He meant the oysters.

As for the communique: That's the agreement issued yesterday by Ronald Reagan and Chun. It affirms U.S. military commitments in South Korea and also establishes that consultations between the two countries will "resume immediately" -- including the security-related meetings withheld by the Carter administration to pressure South Korea on human rights.

So last night's party, given in a moodily lit ballroom of the Shoreham Hotel by Korean Ambassador Yong Shik Kim, amounted to a lot of gleeful back-patting between the South Korean and Reagan administration offcials who came. Everybody seemed to be in a fine mood.

Perhaps in the finest mood of all was Secretary of State Alexander Haig.

He arrived a little late, but soon found himself in the middle of an impromptu press conference near the door. "I think it's an excellent communique," he said. "I like it because I had a hand in drafting it." The U.S. forces in South Korea, he added, "are the bona fides of Amrerican commitment -- not just to Korea, but to all of East Asia."

Haig left not long after this statement. He was not seen talking to Allen, the man who has kept a low profile these days in comparison to the more dominant secretary of state. Asked about his more noticeable role in foreign policy, Haig replied:

"That's because I have a loud voice. I'm basically shy."

Allen, asked about how he's getting along with Haig, replied:

"Who?"

He was asked the question again. This time:

"Spendidly . . . the secretary of state is the principal adviser to the president of foreign policy. What we're facing is a unique historical situation in Washington, in that you have a secretary of state who agrees with the president."

By this time, the long receiving line to greet Chun was winding down. The South Korean president is a former general who seized power in a coup about a year ago, but has ended his country's martial law and commuted the death sentence of Kim Dae Jung, a political dissident.

Critics still say new laws give his government as much control as before, but last night, few of those partaking from one of the more astounding arrays of Washington party food had anything but glowing comments about Chun.

The party's host, Ambassador Kim, was particularly glowing, and particulary about the communique. "Our security problem is very important," said Kim. "Now, the United States and Korea can return to a traditional firm tie between two countries. This means a lot. And not only the security matters, but also the economic cooperation between the two countries."

Chun himself was not available for comment, due to velvet ropes and serious-looking security guards. And unlike the American president, who answers reporters who shout questions at him en route to the ear doctor or a party, Chun usually does not. It is not though proper to shout, one of the Korean press offcials said last night.

Among the guests you could find at the party: White House press secretary James Brady, Sen. Strom Thurmond (R-S.C.), undersecretary of state James Buckley and Saudi Ambassador Faisal Alhegelan.

There were three endless tables of food. One held things like goose liver pate, another held sliced kiwis that you could dip in chocolate fondue, and a third held the oysters and Beluga caviar. That went early.

So some of the guests stayed late, to eat. It was worth it. In fact, the wife of one highly placed White House official was even observed sticking an index finger in the whipped cream.