Slugging pitcher Shohei Ohtani made his major league debut on Thursday as a hitter and singled in his first at-bat but the Oakland Athletics erased the celebratory mood with a 6-5 walk-off victory over the Los Angeles Angels.

The 23-year-old Japanese, who was a spring training invitee on a minor league contract, started as a designated hitter, was booed by the A's home fans when he came to the plate, but connected to right field off right-hander Kendall Graveman in the second inning, advancing Andrelton Simmons to second base.

"I have to say I was happy," Ohtani said when asked how he felt when he was named to the starting lineup.

"I was ready to swing at the first pitch. Simmons was on first base so that helped me hit into the outfield. Teamwork helped create good rhythm in the first half of the game," he said.

Simmons then scored on a ground rule double by Martin Maldonaldo to cap a two-run frame.

Baseball: Ohtani called up for MLB debut on April 1

With his parents and agent Nez Balelo watching from the stands at Oakland Coliseum, Ohtani grounded out in his next three at-bats and struck out swinging in the 11th, before the Athletics' Marcus Semien hit a walkoff single off Noe Ramirez (0-1) in the bottom half to end it.

After the game, Ohtani said he knew his first at-bat would be an unforgettable one but he was too caught up in the moment to think objectively when he reached first with two out in the second inning.

"The game had just started so I was too focused to let the excitement sink in. I'm going to give that ball to my parents," he said.

When reporters reminded him exactly five years ago on March 29 he started his professional career with Nippon Ham, he said he had no idea. He started in right field in that game and had his first pro hit.

Ohtani, who hit .125 in 11 games and struggled in spring exhibition play, is scheduled to make his first start on the mound on Sunday in the finale of the Angels' season-opening four-game series against the Athletics.

As a pitcher, he also had a rugged spring training, giving up 19 runs in five games.

Ohtani signed with the Angels in December after five seasons with the Fighters in Nippon Professional Baseball's Pacific League. Dubbed "Japan's Babe Ruth," he is hoping to test his two-way talent in the majors.