Bill Gates, left, accompanied by Sharon Osberg, is presented with a medal of the American Contract Bridge League. (Mat Hayward/Getty Images)

Most of the entrepreneurs I write about use business as a way to pursue their passions, whether it’s philanthropy, culture, sports or politics.

Sharon Osberg is the other way around.

Her passion is playing bridge, a card game for math whizzes that led her into a rarefied world most others would kill to be a part of.

Osberg parlayed a gift for the game into a series of business opportunities and high-powered “elephant bumping” that includes Warren Buffett and Bill Gates, billionaires whose net worths clock in at $74 billion and $90 billion, respectively.

“Bridge is my world,” said Osberg, who lives in Marin County outside San Francisco. “Everything in my adult life is a result of bridge, one way or another.”

Investor Warren Buffett plays bridge with Berkshire Hathaway shareholders in Omaha, Neb. (Nati Harnik/AP)

Osberg even makes money from the game. She owns a piece of an online bridge company called Bridge Base that earns her a dividend and gives her a rooting interest in the game’s popularity.

Indeed, she has been a bridge teacher and partner to both billionaires. Recently, she partnered with Gates — the founder of Microsoft — at a Toronto bridge tournament where they took second and then relaxed over a glass of wine and chips.

“In photos of Bill and Warren playing bridge,” she said. “I am always the one whose back of the head is facing the camera.”

She calls Gates and Buffett “solid” bridge players and plays with them regularly. “They are not in the upper echelon of national or world players,” she said. “They are very solid, everyday players.”

“I love the game, and I love my partner,” Buffett said. “She’s a fabulous teacher, extremely smart and very patient. They talk about bridge partners who were asked how they should have played their hand, and the partner says, ‘Under an assumed name.’ Sharon doesn’t do that.”

Still, the way each tackles the game may offer a clue to how they do business.

“There’s a big difference between Bill’s and Warren’s approach to learning the game,” Osberg said. “Bill is very scientific. He reads and studies on his own. Warren enjoys playing. Warren has good instincts.”

“When I first met Warren, his game was ragged around the edges,” she said. “We would play in the evening, and I would go through teaching points. He absorbed it like a sponge. Bill is the same way. Pretty big brain capacity.”

No kidding.

Osberg has her own chops in business. She spent 18 years at Wells Fargo, the San Francisco bank that is one of Buffett’s biggest equity holdings. (She was at Wells Fargo before she knew Buffett.)

I called Osberg while researching a story on a mutual fund I am writing about. Our conversation quickly turned toward bridge, Buffett and the interesting world she inhabits thanks to the game she loves.

Some people have paid millions just to have lunch with the Oracle of Omaha. Osberg trades gossip with him on the phone and plays bridge remotely with him three to four times a week.

She attends annual meetings in Omaha of Berkshire Hathaway, the sprawling conglomerate Buffett built. On Sundays, after the meeting winds down, Osberg will play bridge with shareholders as “part of the weekend experience. I play with everybody.”

Buffett recalled one hilarious moment in particular.

“I have a younger sister, Bertie, who likes to play bridge,” Buffett recalled. “She happened to be in [Omaha] with her husband for our annual Berkshire meeting. So the four of us play. By some miracle, my sister and her husband beat me and Sharon. My sister reached for the score pad, so I tore the sheet off and ate it. Not that it is a competitive game or something.”

Osberg has also bumped with the swells in Buffett’s orbit: She was an occasional guest at the Georgetown home of Katharine Graham, then the owner of The Washington Post, when Osberg visited the city during the 1990s.

This actually has an investing component to it: The subculture of bridge goes beyond Forbes billionaires, reaching into executive suites and boardrooms. Hedge fund star David Einhorn is a tournament bridge player. Bear Stearns, the investment firm that failed in the 2008 crash, was known as “the bridge firm” because its top management and many of its quant geeks were players.

“I just kissed [former Bear Stearns chief executive] Jimmy Cayne on the cheek last week,” Osberg said.

Famed value investor and Buffett mentor Ben Graham reportedly compared the strategy of bridge to the discipline of long-term investing.

This is from a 2013 report in the Globe and Mail in Toronto:

“As Graham pointed out, playing your hand right — in bridge or in the stock market — generally leads to success in the long term. It doesn’t, however, guarantee you success right now. Sometimes, playing a hand the right way leads to failure; sometimes picking a stock for the right reasons results in a loss.

“Bridge can teach an investor the importance of sticking to a well-thought-out strategy.”

Osberg is a member of the elite echelon of world-class female players, but she said she is playing in what is widely considered a man’s game.

Bridge has taken her to Tokyo, Athens, Chile, Australia, the island of Corsica, Verona, Paris, Montreal and virtually every major U.S. city.

Bridge is not for the faint of heart.

“Everyone loses more than they win,” Osberg said. “Losing is much more common. You have to develop a thick skin.”

“It’s not easy to sit down to play in a tournament,” she said. “The way you move your cards and how you do your bidding, it’s very difficult.”

She recalled Buffett’s first bridge tournament, held in Albuquerque. They made it to the finals after two grueling qualifying rounds. “That was miraculous,” she said.

But Buffett, the steely capital allocator who moves world markets with mere utterances, had enough.

Osberg recalls: “He said, ‘I can’t do it anymore.’ It was so stressful, he didn’t want to play in the finals.”

“I had no business being in it at all,” Buffett said. “We were playing people not as good as Sharon was, but a whole lot better than I was. I dropped out. I was on the board of USAir at the time, so I said I had to get back to a board meeting. This was not great behavior on my part. I love the game, but playing in tournaments is too many hours of concentration.”

At her peak, Osberg was one of the top players in the world.

“I am no longer a serious player,” she said. “I used to play just to win. Now I play for the beauty of the game. It’s the same way mathematics can be beautiful. Your brain has to be nimble enough to recompute on the fly when information comes in.”

Honestly, I have never thought of math as beautiful. But then Buffett and Gates, not me, are the geniuses traveling in the Gulfstream jets with their bridge maven Osberg.

Osberg grew up near Philadelphia in an upper-middle-class family of Italian immigrants. Her father was a businessman who helped run a family meat business.

She learned to play bridge at Dickinson College in Carlisle, Pa., where she graduated with a political science degree in the early 1970s.

“Somebody a couple of doors down in the dorm said, ‘We need a fourth for bridge.’ I said, ‘I will do it.’ ”

She was a natural. After graduation, Osberg moved to California and joined the large community of bridge players around the Bay Area.

“Everyone knew each other,” she said.

A fellow bridge addict hooked her up with Bank of America, where she participated in a three-month technology training program that introduced her to a business that drew on the same skill-set that made her so successful at bridge.

“I loved programming,” she said. “It’s numbers, pattern recognition, problem-solving. It’s just so cool. The same reason I love bridge.”

Osberg was in the right place at the right time. Bank of America was just beginning to pioneer technology that would lead to online banking.

She eventually spent 18 years at Wells Fargo, where she rose to executive vice president for technology and retired in 2000.

Her years running technology at Wells Fargo brought her into contact with Gates and other technology wheels. She remembers one meeting with entrepreneur Marc Andreessen, co-author of Mosaic, the first widely used Web browser.

The stiff bankers shed their jackets, dress shirts and ties to make their Silicon Valley guests comfortable, only to see Andreessen and company show up wearing suits.

She was invited to New York to play in a bridge tournament in the early 1990s and was partnered with Buffett confidant Carol Loomis, at the time a Fortune magazine writer.

Buffett was playing, too, and invited her to stop in Omaha some time. “I said, ‘Where is Omaha?’ That was not the thing to say.”

When she finally stopped in his hometown, they went to dinner, and Buffett pulled out a blank map of the United States and asked her to draw an X for Omaha.

They quickly became close friends. As her mentor educated her about business and managing people, Osberg bought her first shares in Berkshire Hathaway — at the then-soaring price of $16,050 — “ a fortune to me.” (The stock today sells for around $259,600. Osberg owns a lot more of it.)

“He opened up a world I never would have been part of without him,” she said. “Once or twice a year, I get to sit back and just listen to Bill and Warren. They talk about companies. They talk about trends. Artificial intelligence. Nuclear proliferation. What the future might hold and the political implications in the business world.

“I don’t know how I got that lucky.”

Just look for the one whose back is to the camera.