Willie Mays hit his first home run of the season during his first game as a New York Met yesterday, driving in the winning run in a 5-4 victory over his erstwhile teammates, the San Francisco Giants.

The fifth-inning blast was the 41-year-old titan's 647th career home run, and it moved him past Mel Ott into seventh place on the all-time RBI list, with 1860.

Mays began the Mets' half of the first inning by walking, and when Giant pitcher Sam McDowell walked the next two batters as well, the stage was set for Rusty Staub, who smashed a grand slam home run to put the Mets ahead 4-0.

In his other two times at bat, Mays struck out and walked. The great centerfielder was playing first base, as he frequently has in the last few seasons, in yesterday's game.

The win widened the Mets' lead in the National League's Eastern Division to three games. The Giants, depressingly last in the West, are now eight games behind the division-leading Los Angeles Dodgers.

Mays donned a New York uniform for the first time since the Giants moved west Saturday, after a deal that brought the Giants Charlie Williams, a minor league pitcher, some cash, and a promise that Mays would have a job when he retired.

When the deal was made. Mays denied rumors that New York would give him a psychological boost, claiming his muscles no longer responded to such young men's inspirations. He was batting 184 going into yesterday's game.

There was no word last night as to whether he had revised his prediction.

The Red Sox lost to the Oakland A's yesterday, as first baseman Duane Josephsen, who had earlier tied the game with an eight-inning homer, made a wild throw to the plate with the bases loaded in the ninth.

Rick Miller also homered for the Red Sox, while Mike Epstein and Reggie Jackson hit home runs for the Athletics on the way to their 6-5 victory.

The Red Sox now hold fourth place in the American League's Eastern Division, five games behind first-place Cleveland.