OAKLAND, Calif. -- The first big league game with an automatic runner in extra innings was decided with a blast rather than a bunt.

Marcus Semien began the bottom of the 10th on second base under the new, unpopular rule and scored on Matt Olson's grand slam, sending the Oakland Athletics over the Los Angeles Angels 7-3 on Friday night.

Olson, whose alert throw from first base nailed Angels free runner Shohei Ohtani in the top of the 10th, connected with one out. The A's gathered around home plate to greet him on opening night.

"It definitely is interesting. There's going to be more excitement," Olson said. "I think it would be a lot tougher to go to 17- or 18-inning games with a guy on second. You'd think at some point somebody's probably going to get a knock."

Major League Baseball instituted the controversial extra-inning runner rule for this season to keep games from dragging on amid a compressed, 60-game schedule that started following a months-long delay because of the virus outbreak.

Ohtani, who'd struck out to end the Angels ninth, wasn't quite ready for it: The broadcast showed him in the dugout wearing a pullover when the game went to extras. Caught off-guard, someone reminded him he needed to go to second and he quickly got organized.

Ohtani tried to advance on Jared Walsh's leadoff grounder but was tagged out in a rundown.

"I feel like we're just going to see some different stuff with that," Olson said.

After Semien took second in the bottom of the 10th, the A's loaded the bases against Hansel Robles (0-1) on a hit by pitch, wild pitch and a walk.

"Let's see what we do," former A's pitcher Dave Stewart wrote on Twitter, then cracked, "I guess the bunt is not an option?"

Then: "We don't need no stinking bunt."

New Angels manager Joe Maddon brought in reliever Hoby Milner and, with a five-man infield in place, Olson hit the first pitch far over the right-field wall.

Maddon was asked how the extra-inning runner rule affected his strategy in the 10th.

"It's interesting for sure -- it's [like] a leadoff double in the 10th, so they got us tonight," he said. "Strategy-wise, we got to the five-man infield. We were ready to play that. Of course, the ball went over our heads. Nothing we could do about that. Everything that we had been working on, the guys were ready for tonight, and we really appreciate that."

According to STATS, Olson became the third player to hit a walkoff grand slam on Opening Day. He joined Sixto Lezcano (1980 Brewers) and Jim Presley (1986, Mariners).

Burch Smith (1-0) pitched one inning for the win.

The game went to the 10th tied at 3 after Jason Castro's tying homer in the ninth against A's closer Liam Hendriks.

The Angels couldn't deliver a win for Maddon in his managerial debut with the team. Maddon joined the Angels following five seasons guiding the Chicago Cubs.

"We're basically playing one-third of a season so that's like losing a three-game series. That's what it feels like," Angels starter Andrew Heaney said.

Many of the Angels, including Justin Upton and Brian Goodwin, took a knee for the national anthem while A's players Khris Davis and Tony Kemp raised a right fist to the air.

Los Angeles opened the season in Oakland for the fourth straight year.

The Associated Press contributed to this report.