Ken Osmond, who played the duplicitous teenager Eddie Haskell on the long-running sitcom “Leave It to Beaver,” one moment a smarmy young man when talking to parents, the next moment a devilish troublemaker when the adults were out of sight, died today at his home in the Shadow Hills neighborhood of Los Angeles. He was 76.

His son Eric said the cause was complications of chronic obstructive pulmonary disease and peripheral arterial disease.

Ken Osmond appeared in all six seasons of “Leave It to Beaver,” 1957 to 1963, one of the most-watched television sitcoms of the era, then reprised the role as an adult version of Eddie in the Disney Channel revival series, “The New Leave It to Beaver,” in the 1980s.

He also guest-starred on other popular television series of the ’50s and ’60s, including “Lassie,” “The Adventures of Ozzie and Harriet,” “Wagon Train” and “The Loretta Young Show.”

But for the baby boom generation drawn into the idealized world of postwar television families, Osmond would always be synonymous with Eddie Haskell, by turns the unctuous and mischievous friend of Wally Cleaver, a strait-laced good guy played by Tony Dow.

The Cleavers represented the classic white middle-class family of the Eisenhower era, while Eddie represented danger in a ’50s kind of way — he chewed gum and wore a jean jacket.

Mostly, he sucked up to Wally’s parents, June and Ward Cleaver, played by Barbara Billingsley and Hugh Beaumont, and then poked fun at them when they weren’t looking. He treated Wally’s little brother, Theodore, nicknamed the Beaver, played by Jerry Mathers, as a useless irritant.

“Oh, good afternoon, Mrs. Cleaver,” was a typical Eddie greeting. “I was just telling Wallace how pleasant it would be for Theodore to accompany us to the movies.”

Viewers knew that having the Beaver go to the movies with them was the last thing Eddie had in mind, and that he would find a way to ditch him.

June would sometimes raise a skeptical eyebrow at Eddie, but for the most part she played along with his obsequious manner and almost never confronted him.

In time, Eddie Haskell became so indelibly associated with Osmond that he found it difficult to escape being cast as an Eddie Haskell type, and he left television and joined the Los Angeles Police Department.

Being typecast was “a death sentence,” he told a radio interviewer in 2008.

“I’m not complaining, because Eddie’s been too good to me, but I found work hard to come by,” he said. “In 1968, I bought my first house, in ’69 I got married, and we were going to start a family and I needed a job, so I went out and signed up for the LAPD.”

As an officer on motorcycle patrol, he grew a mustache to disguise himself. In 1980, he was shot three times in a chase with a suspected car thief, but escaped serious injury; one bullet was stopped by his belt buckle, the others by his bulletproof vest. He was put on disability and retired from the force in 1988.

Kenneth Charles Osmond was born on June 7, 1943, in Glendale, California. His father, Thurman Osmond, was a studio carpenter and prop maker, and his mother, Pearl (Hand) Osmond, was an agent who started taking him to auditions when he was 4.

He soon began appearing in commercials. His first speaking part came at age 9, when he was cast in the film “So Big” (1953), starring Jane Wyman and Sterling Hayden. Then came parts in the movies “Good Morning, Miss Dove” (1955) and “Everything but the Truth” (1956).

Osmond grew up in North Hollywood and attended North Hollywood High School. After graduating in the early 1960s, he started a helicopter charter company with his brother, Dayton. When he was completing the last season of “Leave It to Beaver,” he joined the Army reserves, which accounted for his hair being especially short in those episodes.

Though he was annoyed at the typecasting, Osmond could not resist reprising the character. In 1983, while still a police officer, he appeared in the CBS made-for-television movie “Still the Beaver,” which followed the Cleaver boys as adults.

That led to the “The New Leave It to Beaver,” a revival series that ran from 1984 to 1989. Osmond played Eddie as a husband and father, while his character’s two sons, Freddie Haskell and Edward “Bomber” Haskell Jr., were played by Osmond’s two real-life sons, Eric and Christian, respectively.

In addition to his son Eric, he is survived by Christian; his wife, Sandy Purdy; and two grandsons. Dayton Osmond died a few years ago.

Ken Osmond wrote a memoir, “Eddie: The Life and Times of America’s Pre-eminent Bad Boy” (2014, with Christopher Lynch), with a foreword by Mathers.

Eddie Haskell has so endured in popular culture that a psychological syndrome has been named after him — the “Eddie Haskell Effect.” Dr. Ronald Riggio explained it Psychology Today in 2011:

“One reason why workplace bullies may not be discovered is because they suck up to the authorities while bullying subordinates and peers behind their backs. Just like Eddie Haskell from the old ‘Leave it to Beaver’ show (who ingratiated himself to the parents while tormenting the Beaver), the bully pretends to be a model employee — but only when the boss is around.”