Pennants are built on dreams. Champions thrive on those sustaining myths of self-confidence that a club constructs about itself. Talent comes first, of course. But then other forces must lend a hand.

Tonight was the stuff of dreams for the Baltimore Orioles, that bargain-basement powerhouse that thinks it can win a world title on a shoe-string.

With a chanting, shook-up throng of 29,400 putting Fenway Park to shame for hoise, old pinch-hitter Pat Kelly crashed a three-run homer off Bob Stanley in the bottom of the 10th for a 5-2 sudden-death victory over the Boston Red Sox.

The Birds greeted Kelly en masse at the plate, pummeling and bear-hugging him as though this were the World Series. Normally stoic Ken Singleton led the crowd's cheers by pumping his index finger in the air.

The Orioles, winners of 24 of 30 games and possessors of the best record in baseball. 27-14, sound a loud message to baseball these days. Whether they really are "No. 1" will take months to determine. However, the vital first step is for them to believe that they are.

That has been done. Tonight served as their seal of self-approval.

The Orioles, you see, were supposed to lose this game-everything conceivable was against them.

"Our pitching staff was decimated," said cohero Steve Stone, who pitched nine innings of eight-hit ball on a night when the entire Oriole bullpen consisted of one able-bodied man-less-than-dependable Tippy Martinez.

The O's staff was a one-day disaster area: scheduled starter Jim Palmer-sore elbow, next start on Sunday; Don Stanhouse-flu, not in uniform; Tim Stoddard-tender shoulder, needs a little rest; Sammy Stewart-81 pitches the night before.

When Stone, throwing the best fast ball of his eight Oriole starts, finally tired after 120 pitches in his battle with the sinker-baller Stanley, there was Martinez to shut out the Sox in the 10th and get the victory.

Whenever Stone and Martinez needed help-and they needed plenty-they got it with gobs of defensive brilliance. At least seven O's plays were circle-'em-in-red gems.

Kiko Garcia save a run with a scoop-and-peg of a ground smash deflected by Stone. Eddie Murray saved another with a dig of a low Billy Smith throw to end an inning.

In the 10th, Singleton, who had flagged down one fly at the top of the right field fence, made a running catch in the corner to prevent a leadoff double. Two batters later, Al Bumbry made a spectacular flat-out sprinting shoestring catch in center. Rick Dempsey ended the inning by gunning out a would-be thief at second.

In short, all three of Martinez' outs outs were the result of his mates' refusal to let him get in trouble.

Perhaps out-of-position third baseman Rich Dauer, filling in for injured Doug DeCines, best symbolized the Birds. He stopped seve grounders at third-not one of them with his glove.

"I didn't need to take the glove out there with me," said Dauer. "Nothing's touched it yet. Call me the Human Bruise."

One bad hop hit Dauer, who does not wear a protective cup, in the groin. As he collapsed face first in the grass, he managed to make a last-gasp throw to first to nail the runner. Although it took him some time to regain his feet, he stayed in the game.

"I had to make the throw or it would have looked even worse," said Dauer, trying to grin.

By the time the O's came to bat in the 10th, all the psychic energy of this game, all the juice of this pumped-up crowd was on their side. The 2-2 past was long since forgotten.

Sure, Jim Rice had singled home a run in the first, and Murray had answered with an opposite-field homer to left in the second. Jerry Remy had scored on a Fred Lynn force play in the third and Baltimore had answered once more as Singleton scored when Lee May grounded into a double play in the fourth.

After that, this game was a defensive war of attrition in which the Sox also sparkled. A Lynn stab off Bumbry in center was one particularly vivid run-saver.

Finally, the Sox, now 1 1/2 games behind the Birds, started to unravel in the 10th. With one out, Remy barely missed flagging a Singleton grounder that ticked his glove. Mark Belanger laid down an excellent sacrifice bunt that he beat out when Butch Hobson's barehanded scoop-and-flip pulled George Scott off first.

When the Sox argument that Belanger had been tagged finally abated, Kelly, the phlegmatic, born-again pinch swinger stepped up. His rising liner sailed over the right fielder's head and barely over the fence.

"Just get over his head, praise the Lord, that's what I was thinking," said Kelly.

"It pays to serve the master. Preach to 'em, Pat," said the jubilant Singleton, joining the revival spirit.

"I love to win 'em like that," muttered May to a teammate in a corner, "so the other guys walk off real slow . . . with their heads down . . . with nothin' to do."

Stone was left with nothing to do but recite all his superstitious changes-necessitated by "my worst spring since I was a rookie. I shaved my mustache so the Sox wouldn't notice I was the same guy they beat last week. I changed my number (from 21 to 32). I changed my windup. I even ate a lucky Tootsie Roll pop."