The Baltimore Orioles' locker room, after the O's 12-2 home Opening Day win over Kansas City today, achieved a mood of such sweet contentment that it was like the peace in a family parlor after a long Thanksgiving dinner.

Any team, after winning a slugfest before a Baltimore opener record crowd of 50,199 joyous fans, would be pleased with itself, would quickly forget the chilly spring winds and the raw midgame rain shower.

The Orioles, however, were happy about more than their dozen hits, five for extra bases, and their 19 base runners. They were pleased by something that ran deeper than a sharp Jim Palmer victory or a jovial Ed Williams, the new owner, passing among them.

The Birds may not be a family, but they are a true community -- one that is at ease with itself and has the knack for sharing its blessings, its sense of being a young, healthy champion at play.

Music and good will, confidence and pleasure at the start of a new season filled the Oriole clubhouse after the first lopsided laughter of the season.

Rick Dempsey, who had hit a single, double and homer, searched the room with his eyes looking for a crippled child. "Hey, Ronnie, come here," said the catcher, slapping hands with the little boy in the big Oriole cap. "How'd you like that?"

Ken Singleton, who had clouted a two-run homer and scored four runs, made sure that he set his bat aside so that the boy could have it. "He's just a little friend of Dempsey's," said Singleton. "He visited us about 20 times last season. "We're all crazy about him.I think he's got some of everybody's equipment."

Many other baseball teams are loud and belligerent. The Orioles play rock 'n roll, too, but theirs is soft and toe-tapping melancholy: "I'll be working my way back to you, babe."

All in all, the afternoon created an atmosphere of subdues celebration -- real gratitude for the return of a game and a special team.

"It's all different now," said winning pitcher Palmer, who didn't allow a man beyond first base until he had a 10-0 lead after six innings. "Opening day used to be a ritual here -- that's all. Now they really come for the Orioles. It's our crowd."

"All those people hollerin' was just what we needed, coming home with a 1-3 record," said Manager Earl Weaver, who could not disguise his pregame jitters over a team that lost its last eight straight exhibition games.

"I've seen nothing in the dugout to indicate that we're a different team from last year -- that we can't light a fire under ourselves when we need to. But the crowd really helped us today."

The previous attendance record for an Oriole opener was 46,425, on April 9, 1976, against the Red Sox. Today's crowd was the fourth largest for a regular-season game in Baltimore.

The O's were as contented after their romp as a team should be that had just seen the American League championship flag raised over its park and had received its title rings.

While several players casually took turns making sure that Ronnie was having a good time ("You stay right here until I get back from my shower," said Dempsey), others were receiving congratulations from Williams, wearing a "Baltimore Is Best" tie. r

"Nice homer," said Williams to Singleton.

"They don't count when you hit 'em when it's 10-2," the team's MVP told the owner, giving a bit of graceful remedial education. "It's a nice homer when I hit one with a man on in the ninth and we're a run down."

Eddie Murray, who, like Lee May, had chipped in a single and double, picked up Spinners album for the tape deck. In every locker room corner, there were tiny knots of congratulation.

"I haven't seen Palmer that sharp since '78," said Mark Belanger, the shortstop with the new stance (again) who had two hits and two fielding gems. "He's healthy. He's happy. And he looks like 20 wins to me."

"Jim threw 101 pitches in his eight innings," said Coach Ray Miller, giving his daily review of the state of the art.

"Of them, 65 were fast balls and 66 were strikes. He had exceptional stuff and exceptional control."

Palmer had more than that. For the first time in a long time, he was making good hitters look very awkward. Instead of the Palmer trend of recent years toward a steady diet of well-struck fly-outs, this game saw him strike out five. Palmer fanned Willie Wilson with a deceitful changeup and whiffed slugger Willie Mays Aikens twice with a succession of fast balls that were such an affront to Aikens that he walked around in small circles of fury outside the batter's box between pitches.

Except for a meaningless solo homer by George Brett in the eighth, not a Royal hit a ball off Palmer that traveled more than 250 feet in the air with any authority.

"Palmer was exceptionally good," said Brett. "With no leaves on the trees in centerfield, his fast ball was just exploding out of those white houses across the street."

Perhaps the best sign of the day for the O's, who still have serious short-term worries over the sore arms of Dennis Martinez and Scott McGregor, was Palmer's attitude of general contentment and conciliation.

Besides kind words for the Baltimore fans who have booed him, he explained, almost guiltily, that he didn't think that he deserved to start two opening days within five days (the other a 5-3 win in Chicago).

"The honor should have gone to Mike Flanagan," said Palmer, who has eight 20-win years and three Cy Young Awards to one of each for Flanagan. "If he'd gotten the runs I have (5-0 and 5-0 leads after two innings in his two starts), then he'd be the one who's 2-0.

"On the other hand," said Palmer, who has survived years of chronic nonsupport by Oriole bats, "maybe it's a new decade. Maybe I'll get the runs in the '80s that I didn't get in the '70s."

On a day when Maryland Gov. Harry Hughes and Baseball Commissioner Bowie Kuhn were on hand to throw out the first ball and the first cliche, respectively, the victim of the first practical joke was Dempsey.

After the well-lubricated fans in Section 34 had interrupted the National Anthem with their explosive rendition of the letter "O," the Orioles lined up on the top dugout step to race on the field for their ovation.

"Okay, let's go," shouted one Oriole, giving the cue to charge.

The peppery Dempsey sprinted to home plate only to turn around and discover that the other Birds hadn't budged, except for the few who had collapsed from laughter.

"Now that Dempsey's convinced he's a slugger," said Singleton, " we have to do little things to keep him in line."

If the Birds today looked like the Orioles who won 102 regular season games last season, it was partly an illusion.

True, the Birds, as usual, chewed up an opponent who made fundamental gaffs. Starter Dennis Leonard walked men with the bases empty in the first two innings and both eventually scored. In all, five of seven Bird walks were turned into runs. Also, an unnecessary throw by Frank White on a unmakeable double play went into the K.C. dugout for a free O's run.

When the Birds led, 4-0, at the game's mid-point, it could easily have been 0-0.

However, familiar as the Bird victory style looked, several O's -- particularly Belanger, Doug DeCinces and Dempsey -- look so different (and improved) at the plate that they almost seem like different players. Belanger and DeCinces are merely trying to turn back time to their levels of '78, but Dempsey . . .

"I've worked with Nautilus equipment all winter, and I've completely changed almost everything I do at the plate," said Dempsey. "I'm so much stronger and so much more relaxed, that I'm having a whole bunch of new sensations."

Sensations like "hitting one off the end of the bat" and watching in disbelief as it digs through a 25-mile-per-hour left-to-right wind and lands in the hands of a first-row bleacher fan for a home run.