Dr. Margaret O'Leary, a resident in internal medicine at George Washington University Hospital, is about to have a baby.

"Please don't have it now," her husband told her Monday night. "I really couldn't handle that."

Margaret O'Leary's husband of one year, Dennis S. O'Leary, was very, very busy Monday, and yesterday and probably today and tomorrow.As dean of clinical affairs at George Washington Medical Center, he mobilized the hospital to meet the challenge of the attempted presidential assassination, and served as the information link to the world.

In the second role, he also became very, very famous, this cool 43-year-old hospital administrator who faced down -- and tamed -- a frustrated and near-ugly press corps waiting Monday evening for the report on the medical aftermath to that afternoon's assassination attempt.

Herded into the GW medical school's Ross Hall Lecture Room 101, reporters waited hours for word on the surgery that removed the bullet from the president's lung and the conditions of the other victims, especially presidential press secretary James Brady, a friend to many of the reporters there.

"I've lectured in that room many times," O'Leary was saying yesterday afternoon, "but when I walked in there last night, it was the most unreal sight, all those lights and instruments and cameras. . . It's difficult . . . a lot different from a lecture . . . when so many people are talking and you try to isolate who it is with the lights in your eyes . . . and some of the questions are hard and some are leading. . ."

None of this uncertainty showed, however, not to the corps of reporters, which can have an uncanny mob sense for such things, and not to the TV public. And when he raised his arm to demonstrate how the bullet had entered the president's body, Dennis O'Leary became a star. Of medium height, slim, his hairline just beginning to show a hint of recession, the mellifluous-voiced doctor has, in two days, become a symbol of calm and reassurance to a nationwide audience.

Describing himself yesterday as so tired he was "scrambled," O'Leary guessed it would be a day or so before his stardom sunk in.

"It's a very different experience," he was musing, "not something you ever think you'll be faced with, or would want to be."

Dennis O'Leary is from Kansas City -- "born in Missouri, raised in Kansas" -- went to Harvard, Cornell Medical School, interned in Minnesota, did his residency in Rochester, N.Y., and took the directorship of the coagulation research laboratory at Walter Reed.

Then, as he puts it, "it was time to go out and get a job."

He came to GW about 10 years ago as an assistant professor of laboratory medicine and moved up into his present position. He has three children from an earlier marriage.

A popular lecturer with the students, he says he "thinks" he is "fairly patient," a quality that stood him in good stead with the news-hungry reporters.

To some of the doctors around the hospital he is considered too much of a bureaucrat sometimes, using such business-executive terms as "bottom line" or "at this point in time" or "let's flush it out."

But he is exquisitely aware of the GW Medical Center's uniquely central positioning, which has made it a focus for crises -- the Hanafi Muslim takeover and then, later, the release of their hostages, for example -- and he keeps his staff "keyed in to crisis management."

"They've never let me down," he says with a coach's pride.

"It was a kind of strange message we got," he says, speaking of Monday's shootings. "During the Carter administration there was installed a direct line to the White House. The call that came in on that line Monday just said that the president's motorcade would be arriving almost immediately.

"The person who took the call might have taught, 'Gee, that's nice, the president's motorcade is coming for a visit.' Fortunately, the person figured there must be some sort of problem. And the president beat the trauma team to the emergency room by only about 30 seconds."

The hospital instantly went into an often-practiced disaster plan, mobilizing the emergency room, moving the most needy patients into rooms, shunting aside those who could wait (some of whom gave up and went home,) but "once he [President Reagan] got in, it ran very smoothly."

"One thing," he says, "it was different from what happened at Parkland [the hospital in Dallas where the fatally wounded President Kennedy was taken] in that we had good traffic control. Only people with good reason to be there were there."

(Reporters were not considered people with good reason and were left out in the rain until Room 101 was made available. Even a buffet of sandwiches, coffee and soft drinks did little to mollify many of the more aggressive journalists and TV reporters, making O'Leary's eventual total conquest even more impressive.)

O'Leary is a crossword puzzle addict, likes racquet sports, hiking, jigsaw puzzles and an 8-month-old dachshund "with really unusual coloring."

In the end it was the dog that kept him from getting more than four hours' sleep Monday night.

"When I finally got home," O'Leary says, with a helplessness only dog owners can comprehend, "he wanted to play."

And what would he have been doing if the week had not, as he put it, "tended to divert one's attention?"

Another sigh.

"I'd like to say . . . well . . . resting."