SANDWICH, England — Louis Oosthuizen had certainly paid his dues since winning the British Open in 2010, finishing as runner-up on six occasions at major championships.Jordan Spieth had paid a few, too, reviving his fading game after two years of struggle.But Collin Morikawa is a young man in a hurry, and on Sunday, he again proved that experience was overrated, winning the British Open on his first attempt by outplaying Oosthuizen in the final pairing and holding off Spieth on the closing holes.“You have to embrace it,” Morikawa, a 24-year-old Californian, said. “You have to be excited about these opportunities, and that’s how I looked at it today, especially coming down the stretch.”Much easier said than done, but Morikawa looked focused yet not too tightly wound from the start: chuckling with his caddie, J.J. Jakovac, as they walked up the first fairway and then holding remarkably firm as the pressure continued to rise on another sun-drenched afternoon at the Royal St. George’s Golf Club.He might not have won a British Open in classic conditions — howling winds, driving rain and summer chill — but he still won it in style. He made four birdies and nary a bogey as he shot a four-under-par 66 to finish at 15 under, two strokes ahead of Spieth, who was playing in the penultimate group and playing very well. “Clearly, with the shots he’s hit and the putts he’s holed, he’s not afraid of high-pressure situations and winning a major championship,” Spieth said of Morikawa.Royal St. George’s is the same venerable and undulating English seaside course where another young American, Ben Curtis, prevailed on his first visit in 2003.But Curtis was a huge surprise who has yet to win another major. Morikawa is an established threat who was ranked fourth in the world when he arrived in Sandwich.Last year, he won the P.G.A. Championship, also on his first attempt, calmly staring down a closely packed leaderboard in the closing holes at T.P.C. Harding Park in San Francisco to claim his first major.Learning curve? What learning curve?“It’s the same thing I said after he won the P.G.A. It seems like he’s been there 100 times, and he just hasn’t,” Jakovac said. “It just goes to his mental strength and his maturity, and you add the freakish ball striking to an absolute stone-cold demeanor who is very comfortable in all the situations — you get somebody pretty special.”Morikawa is the first player in men’s golf to win two different major championships in his first appearance at them.He graduated with a business degree in 2019 after four years at the University of California, Berkeley. He is not the longest hitter and has sometimes struggled mightily with his putting. His dreamy long-iron play is the core of his game. Slow backswings are his hallmark. He is a deliberate player — the unexamined shot is definitely not worth hitting — but he clearly has a knack for preparing himself mentally for the game’s biggest challenges.“I’ve had belief in myself from Day 1 that I turned professional that I can do it,” Morikawa said.“When it comes to these tournaments I’ve never played, I do my work, do my homework Monday through Wednesday to know what I have to do.”With little experience on links courses, he went to the Scottish Open the week before arriving at Royal St. George’s. He decided to change his 7-, 8- and 9-irons before the start of the British Open. On Wednesday, the day before the tournament began, he decided to revert to a conventional putting grip for long-range putts to get more power.It worked, and though he ranks near the bottom of the PGA Tour’s putting statistics this season, he putted beautifully under pressure, particularly on SundayBut while deep research and smart changes clearly have their benefits, it is still difficult to model outcomes like Sunday’s when you are in the final pairing of a major trailing a former champion like Oosthuizen by two shots at the start of the round with 32,000 fans gathered outside the ropes and ready to roar from the knolls and ridges.He won the P.G.A. Championship with no fans on site because of the pandemic.“I hope the thing is off the table that I can play with fans and I can play well on a Sunday,” Morikawa said.He took it all in on this particular Sunday and did not crack. Instead, it was Oosthuizen, the 38-year-old South African, who failed to close despite leading throughout much of the tournament.Morikawa took a share of the lead at the fourth hole and took the outright lead for the first time in the tournament on the par-5 seventh, knocking in a tap-in birdie after Oosthuizen had hit his approach shot into a bunker and then mis-hit his next shot into another bunker across the green. Oosthuizen finished with a bogey 6: a two-stroke swing in what felt, at that stage, like match play.Morikawa never relinquished the lead despite an inspired charge from Spieth, the 27-year-old American who won the 2017 British Open.After a shaky start, Spieth made an eagle on the seventh and got to within one shot of Morikawa’s lead when he sank a birdie putt on the 14th hole to go to 13 under.But Morikawa, well aware of Spieth’s surge, quickly gave himself more cushion by making a curling uphill birdie putt on the 14th hole to move to 15 under. Spieth could not manage to close the gap despite all the spring in his step and urgency in his words as he talked to his shots in midair.Morikawa closed out his latest remarkable major by making par on the final four holes, with the only genuine scare coming at 15 where he hit his approach shot into deep rough. But he chipped to within 10 feet and then made the putt. He finished with a 15-under-par total of 265 after receiving a standing ovation from the full grandstand on the 18th hole. Spieth was second at 13 under. Oosthuizen and Jon Rahm, this year’s U.S. Open champion, tied for third at 11 under.“I’m glad I look calm because the nerves are definitely up there, but you channel these nerves into excitement and energy,” he said. “That’s how I look at it, especially as those last nine holes were coming in. Jordan was making birdies. I think Rahm was pushing. Louis had a birdie on 11, an amazing birdie. You can’t worry about the score. I had to worry about every shot. Can I execute every shot to the best of my ability? Some we did, some we didn’t, and then you move on.”Morikawa is only the second player to win twice in his first eight majors. The other was Bobby Jones in 1926. For reference, it took Tiger Woods 18 starts to win his first two majors. That is not to imply that we should be rushing to compare Morikawa to Woods, a 14-time major champion who became one of the biggest stars in global sports. But Morikawa has major dreams of his own.“I think he’s got some big goals in mind for his career in golf,” Jakovac said. “You should be more worried about him not enjoying this enough. He is more of a ‘let’s go get the next one’ kind of guy.”Was he already talking about the next one on Sunday?“No,” Jakovac said. “But he did after the P.G.A. way too soon. I was like, ‘Bro, just relax.’”Morikawa seems to have gotten the message.“At 24 years old, it’s so hard to look back at the two short years that I have been a pro and see what I’ve done because I want more,” he said. “I enjoy these moments, and I love it. And I want to teach myself to embrace it a little more, maybe spend a few extra days and sit back and drink out of this.”He was referring to the claret jug, which is awarded to the winner of the British Open and which already had his name engraved on it as he held it close on Sunday night.