Bobby Valentine, the rookie manager, promised that the Texas Rangers, the rookie team, would be different.

Different as in five rookie pitchers, and a rookie right fielder who never had played in the minors. Different as in a coaching staff that averages 38 years of age.

Different as in coming to Memorial Stadium, where they'd won only five times in five seasons, and pounding out three home runs and a 12-3 victory over the Baltimore Orioles before 17,545 tonight.

At 6-3, the Rangers not only lead the American League West for the first time since July 17, 1983, but are tied with the New York Yankees for the best record in the American League.

They got 15 hits off three pitchers, with the biggest of those coming from Pete O'Brien, whose three-run homer in the fifth ining broke open a close game and knocked out Scott McGregor (1-1).

They also got a bases-empty homer from third baseman Steve Buechele and a three-run homer from right fielder Pete Incaviglia, his second homer of the season. Incaviglia is trying to make the jump directly from Oklahoma State University, and although he's hitting .114, has two homers against the Orioles.

In all, the Rangers crushed a pitching staff that had been far and away the best in the league, entering the game with a 1.98 ERA.

"It's going to happen sometimes," Orioles Manager Earl Weaver said. "You're going to have about 10 of these a year. It's a shame for the fans, but we'll do it to someone else a number of times. At least, I hope we will."

McGregor had beaten the Rangers Sunday and skipped through the first four innings tonight, allowing a run on two singles. But he ended up yielding seven hits and five runs in 4 2/3 innings, only the second bad performance by an Orioles starter this season.

"I just threw too many balls down the middle," he said. "You can't do that. I just didn't have much control. Unfortunately, I'd pitched four good games in a row, but wasted two of them in Florida."

He said the pitch to O'Brien was "a hanging curveball," which is the worst kind.

Meanwhile, the Rangers got seven survivalist innings from left-hander Mike Mason (1-0), who scattered 10 hits to beat the Orioles for the first time in his career. Baltimore had early leads of 1-0 and 2-1, but the Rangers got him six runs in the fifth and four in the sixth for an 11-2 lead.

"Good defense and a lot of runs," Mason said. "The price was right."

The good news for the Orioles was that first baseman Eddie Murray went three for four, and, with five hits in two games, is out of his slump. In two games, he has raised his average from .094 to .200.

In all, the Orioles got a season-high 11 hits, but hit into three double plays and went without a home run for the seventh of 10 games.

"We're hitting the ball, but now we've got to relax a little with men on base," Weaver said. "We could have broken a few games open with the right hit here and there. We're not getting the biggie yet."

The Orioles are hitting .193 with runners in scoring position, but in a 5-5 start their pitching has been good enough to make up for that.

Meanwhile, the Rangers scored 10 or more runs for the third time this season after doing it only five times all last season.

The Orioles got a 1-0 lead in the second inning when Murray singled into right field, went to second on Juan Beniquez's single and scored on designated hitter Mike Young's double to center. Mason struck out Shelby and got Rick Dempsey on a grounder to third to end the inning.

The Rangers tied it in the third inning when Don Slaught led off with a single to left, and McGregor hit Tom Paciorek. Buechele flied to center, but Scott Fletcher singled to center to score Slaught.

Wiggins' speed gave the Orioles a 2-1 lead in the third. He led off with a walk and stole second. Lee Lacy popped out, but, with Ripken at the plate, Wiggins stole third, the Orioles' ninth steal of the season.

Ripken followed with an RBI single to center.

But the Rangers broke it open in the fifth by sending up 11 hitters and getting seven hits to take a 7-2 lead. Slaught and Paciorek singled, and, after McGregor got Buechele and Fletcher on flyouts, Harrah looped a broken-bat single into right to score Slaught.

McGregor then hung the curveball to O'Brien, who put it into the right field seats for his third homer and a 5-2 lead.

Weaver went for Dennis Martinez, who was making his first appearance of the season and pitched like it was, facing four batters and allowing a walk and three singles.

Murray will add a new touch to his wardrobe Saturday night, when he will wear glasses for the first time in his career. He's going to try them (if they're ready) because doctors detected a mild weakness in his right eye . . . The Rangers are 6-19 in Baltimore the last six seasons . . .

The Orioles have committed a league-high 12 errors and are last in the AL in fielding . . . Third baseman Floyd Rayford accepted his rehabilitation assignment to Class AAA Rochester tonight and will play games there Saturday and Sunday. If he has no more problems with his injured left thumb, he'll join the Orioles Monday in Cleveland . . . The Rangers' 12 runs were the most they've ever scored in 160 games against the Orioles.