If the Supreme Court justices had to rule on whether Tim O'Brien should be named their favourite reporter for 1979, the decision probably would be 9-0 against him.

O'Brien is the ABC-TV corespondent who last month reported in advance the Supreme Court ruling that public figures suing journalists for libel could force reporters and editors to reveal the thoughts, opinion and conclusions they had while while preparing the contested story.

O'Brien's report led to an internal investigation by Chief Justice Warren E. Burger's aides to pinpoint the source of the leak. And O'Brein's colleagues - despite what he says are telling him - are furious that he reported the decision two days before the court announced it.

"It was cowboy journalism," remarked one court correspondent who, like other reporters who work with O'Brien, prefers to remain unnamed lest he be accused of sour grapes. "You have to be pretty desperate before you start going coast-to-coast on the air with these premilinary matters. I don't want to suggest we've been co-opted into any kind of system of restraint, but we're in a profession where we have to be right 100 percent of the time. Unless you can confirm it, you can't go with it."

Says another court regular: "It doesn't get us anything. It doesn't mean anything. And the possibilities for corruption are limitless."

Predicts another: "We're going to have to see if the ground rules have changed, There'll be a real shakedown over the next couple of weeks."

Columnist James Reston blasted ABC's decision to beat the court's public announcement as a "drop of poison in the whole democratic process."

The focus of these invectives is a 35-year-old, enthusiastic, boyish-looking reporter who is the junior member of the trio television network correspondents (all of them lawyers) who cover the Supreme Court. O'Brien says he has received universal approval" from colleagues.

"We all try to distinguish ourselves not by being first but by being best," says O'Brien, who followed his first exclusive the next night with a second, reporting that the court would reverse a lower court opinion that state prison inmates awaiting parole must be afforded certain due process protections. And the ABC reporter gives the impression to an interviewer that he could keep coming up with advance news of court rulings all summer long. (He won't comment on whether the printer fixed for suspected leaking was a source of his.)

"Each case is being considered individually," he says of the process by which he and his superiors decide whether to air decisions in advance. What factors are taken into consideration? The quality of the information, he says, and a less tangible consideration: how mad he might make the justices. O'Brien says he doesn't want to go around "frivolously aggravating people. That's a consideration, though it's not conclusive."

The difficulty with reporting a Supreme Court decision in advance is that until the ruling is announced, a justice has the right to change his vote. And court reporters say gossip and predictions are cheap. Just as Capitol Hill staffers trade predictions on vote on major issues, so do court employes talk loosely about the likely vote on hot decisions.

O'Brien says he and his superiors held long conversations on whether to go with his first stories. "I feel that it's our functions to report news as we know it," says O'Brien, "not to keep the court's secrets for them. CBS and NBC would have gone with story in a minute if they had it. They probably would have led with it."

O'Brien remembers pacing at 3 a.m. around the house his family recently bought in Kensington (he has a 4-year-old son and a 6-year -old daugther), asking himself: "What if? Why do I put this house on the line?" His wife worried about the stakes involved, he worried about putting his network's credibility on the line. His job could also have been on the line because, unlike many other network correspondents, O'Brien dosen't have a contract.

What if he'd been wrong?

"Curtains," says O'Brien.He had be singing country andwestern music in Biloxi."