OAKLAND -- Matt Olson had a feeling a showdown with Angels left-hander Hoby Milner would materialize at some point on Friday night, he just didn’t know the stakes would be so high.

A walk to Khris Davis set the stage as the bases were loaded with one out in the the 10th inning of the first extra-inning showdown under the new rules for the 2020 season. With Olson coming up, Angels manager Joe Maddon countered by bringing in Milner to face the left-handed-hitting first baseman. That strategy backfired, as Olson unloaded on a first-pitch slider for a grand slam as the A’s walked it off with a 7-3 victory on Opening Day at the Coliseum.

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Olson had been studying film on Milner prior to the start of the opening four-game series. He was well aware of the lefty’s tendency to try to get ahead in the count with a slider and had a feeling the pitch was coming. The celebration was fairly tame for a walk-off, and Olson said that may have had something to do with the fact that he played out the scenario in his head just before he walked up to the plate for the at-bat.

“I don’t really know what the reaction was,” Olson joked of the celebration. “I had seen the video and was going in sitting on that slider. I was visualizing it in my head. When it happened, I was chill about it. But I was pumped up.”

Olson’s heroic feat marked the first time a player had hit a walk-off grand slam on Opening Day since Jim Presley of the Mariners did it in 1986.

“He’s got a flair for the dramatic,” A’s manager Bob Melvin said of Olson. “He had a good idea of what he was looking for up there, and he got it. He was just trying to get the ball in the air, and he did more than that. We’ve seen Oly walk off against some pretty good left-handers. We always feel good about him in those situations.”

It didn’t take long for baseball’s new extra-inning rule for 2020 -- which dictates that every half-inning in extras begins with an automatic runner on second base -- to come into play, and Olson was right in the middle of it.

Before Olson’s clutch hit, the Gold Glove first baseman provided a brilliant defensive play in the top half of the inning. The Angels placed Shohei Ohtani at second to begin the 10th, but that advantage was quickly nullified by a heads-up play from Olson, who snared a hard grounder by pinch-hitter Jared Walsh and fired the ball over to Matt Chapman at third to catch Ohtani in a rundown for a 3-5-6-5 putout.

The play was something Olson and Chapman had worked on in the past, not necessarily planning for the new extra-inning rule, but for any time when a left-handed batter was at the plate with a runner on second. Sure enough, the left-handed-hitting Walsh was at the plate to begin the inning.

“Those two have been talking about that play for a long time,” Melvin said. “It didn’t surprise me. Two terrific defenders made a great play. That was a game-saver.”

Olson said he wouldn’t be surprised if the need for such a play arises again during the season.

“We talk about it a bunch,” Olson said. “I feel like [Chapman] is athletic enough to get over to the bag, and I have a good enough arm to get it across for an out. Going forward, this is something that’s going to be open to do.”

There was no crowd at the Coliseum to spur on the A’s, but they summoned some late-inning magic nonetheless.

After a rare error by Chapman in the seventh led to the Angels taking a 2-1, the A’s third baseman made up for his mistake in the eighth as he banged a go-ahead triple off the right-field wall to put the A’s ahead, 3-2. Chapman’s clutch hit scored Ramón Laureano, who had just tied the game with a double.

But the late-inning heroics erased what had been a tough go for the A’s offense, which was limited to a solo shot by Laureano off Angels starter Andrew Heaney in the fourth that was clubbed with an exit velocity of 110.5 mph and traveled 441 feet, according to Statcast.

Coming off a year that saw him earn the first All-Star selection of his career, A’s closer Liam Hendriks was unable to secure his first save of 2020, as he surrendered a game-tying solo blast to Jason Castro in the ninth. Despite the blown save, Hendriks regrouped and was able to escape a bases-loaded jam with a strikeout of Ohtani to end the inning.

A high pitch count limited Frankie Montas to four innings, but he impressed nonetheless in his first career Opening Day start. The right-hander held the Angels to one run in four innings, showing off a fastball that sat around 97-98 mph for most of the night as he registered five punchouts.