Frank Sinatra was the King of the Rat Pack — a charismatic bad boy, known nearly as much for his swagger as he was for his golden voice. But one of his closest friends, Tony Oppedisano, saw a side of Ol’ Blue Eyes that was kinder — and more compelling — than the legend.

“Most people would think he would say his greatest accomplishment was one of his huge records like ‘My Way,’” Tony tells Closer in an exclusive interview. “But I think [he’d say] his greatest accomplishment was trying to … help as many people as he possibly could, whether he knew them or not.

Tony ought to know. He was in Frank’s inner circle for decades. A former jazz musician, he first met the crooner in 1972 through a mutual friend, nightclub owner Jilly Rizzo. As they grew closer, he went on to become a member of Frank’s management team — as well as one of his most trusted confidants.

“He taught me some of the greatest life lessons,” says Tony, who details his experiences with the Chairman of the Board in the new book, Sinatra and Me: In the Wee Small Hours. “He was like a second father to me.”

Rest assured, Tony says, Frank treasured his own children, Nancy, 81, Tina, 72, and Frank Jr., who passed away in 2016 at the age of 72. “He went out of his way to be the best father he could,” Tony recalls.

And he tried hard to protect them. “It was in his DNA from growing up,” Tony says of Frank’s natural instinct to take care of those who needed it — something that extended beyond family. He’d help out his friends “emotionally and financially,” says Tony, even paying for life-saving surgeries.

Frank was also there for his onetime girlfriend Marilyn Monroe (“He felt that she was … almost like a frightened little girl”), and his third wife, Mia Farrow, for whom he remained a strong shoulder to cry on long after they split.

The most surprising thing about Frank? He couldn’t stand conflict. “As tough a guy as he was — and God bless him, he grew up in a tough neighborhood — when he got into any kind of conflict with someone he cared deeply about, like his kids or [his fourth wife] Barbara, he would almost become a little kid and try to make the world go away,” Tony relates. “He didn’t like to argue with people he loved.”

Decades after leaving his first wife, Nancy, for Ava Gardner, Frank still felt bad about the hurt he caused her. “We were on a flight overseas once, and everyone on the plane was asleep,” Tony recalls. “He was talking about the night he won the Academy Award for From Here to Eternity and how, when he went back to the house, Mrs. Nancy had organized a gathering for him. He … felt overwhelmed, and he rushed out. He regretted that.” Indeed, Frank was a surprisingly sensitive soul, right up to the very end. Says Tony: “I miss him as much as my own father.”