The Oakland A's taught the New York Yankees a thing or two about bully ball today. They took Ron Guidry uptown, downtown and out of town on back-to-back home runs in the third inning. And the A's beat the Yankees for the second day in a row, this time 6-3, to become the first 20-game winners in the major leagues.

Rob Picciolo, who hit five home runs in 1980, put the A's ahead for good with his second of 1981. Smiling sweetly, he later denied using a corked bat.

Cliff Johnson, who had a two-run homer in the seventh, came to Picciolo's defense. "The cork," he said, flexing his muscles, "is here."

No doubt about it, the A's are flexing their muscles. They lead the league with 27 home runs, including the four they hit today. So much for Billy Ball.

The Yankees, who came into the series batting .220 with 19 homers, bombed out today, leaving 11 men on base.

It began innocently enough, with Tony Armas putting the A's ahead, 1-0, in the bottom of the second with his eighth home run this year. And the newly installed sound system blared "Celebrate Good Times" for the first time today. But the crowd of 48,179 that filled Oakland-Alameda County Stadium had no idea just how good its game would become.

In the top of the third, Matt Keough, who was about as dour as a man about to become a 5-0 pitcher can be, walked Bucky Dent and Willie Randolph bunted safely past Johnson, who is a still life first base. After the runners were sacrificed ahead, Dave Winfield delivered a $20 million single to tie the score.

Up stepped mighty Reggie Jackson. The crowd booed its former idol. When Keough, long suspected of throwing unsanitary pitches, fell behind, 2-1, on the two pitches that broke way away, Jackson exercised his constitutional right and asked umpire Jim McKean to inspect the ball.

At 3-1, Jackson jumped in front of the plate, his arms raised in disgust and despair. Keough, who insists he is as innocent as he looks, almost hit Jackson with the ball as he flipped it back to the umpire.When Jackson walked on the next pitch, filling the bases, he walked to first, licking his fingers in disdain.

But Oscar Gamble struck out on a high fast ball and Graig Nettles flied out to right field and the Yankees were left licking their wounds.

"I just wanted to let the people know I was glad to be home," Jackson said. "If the umpires aren't gonna call it, and you guys aren't gonna do anything about it, I'm just part of the crowd. He's a super pticher, with super pitches. It's a great team and a great town. I like it here."

Yankee Manager Gene Michael said, "If the league wanted to stop it, they could. Any umpire with good eyesight can stop it."

Matt the Martyr said, "They can ask for the ball all they want. Enough has been said about it."

But the A's had not had enough. They scored three runs in the bottom of the inning when Guidry, who had surrendered two home runs all year, gave up two well-met homers to Picciolo and Rickey Henderson and an RBI single to Tim Hosley.

Guidry (2-2) left after 2 2/3 innings, his shortest-lived starting assignment since last July.

Kenough was less than pleased with himself for allowing the Yankees to load the bases in the eighth (and they put two on in the ninth, ending Keough's stint an out short of a complete game). But with two out, and one run in, pinch hitter Bobby Murcer struck out.

The pitch? "A high spitter," Murcer said. "No, really, he threw me a curve."

Which is what Billy Martin and his A's have thrown at the rest of the division.