In an emotional deathbed vow, big-hearted rocker Ozzy Osbourne kissed a single mom stricken with cancer and told her he'd fund every need of her teenage son, family members said yesterday.

Osbourne and his wife, Sharon, made their dramatic promise exactly one week before the death of Reagan Erin Marcato, the mother of Osbourne family friend Robert Marcato, 18. 

The woman's sister Russelle Marcato Westbrook, 39, dabbed away tears yesterday as she recounted to The New York Post the final days of Reagan, who succumbed to colon cancer on July 30 at age 36. 

"She was a great, great sister," Westbrook said. "She was not ready to die. She fought like a tiger, like a champion." 

Marcato gained comfort in her dying days, surrounded by family and friends and receiving regular visits from members of the Osbourne family. 

Ozzy Osbourne leaned over Marcato as she lay dying, kissed her and guaranteed that her son will never have to worry about money or tuition, the mom's sister said. 

"He said, 'Don't you worry about a thing; he's my son now,' " said Westbrook. "The family's grateful." 

The Osbourne family grew particularly close to Marcato this summer when Sharon also was diagnosed with colon cancer. 

In an exclusive interview with The Post on Tuesday, Sharon Osbourne said her family was going to make sure Robert -- whom she called "Baby Osbourne" -- goes to college or drama school. 

"Sharon promised her that," an appreciative Westbrook said. "She said, 'Financially your son will never worry for anything. We're going to put him through college, even if Ozzy has to drive him there everyday himself.' " 

The Osbournes are stars of their own profanity-laced reality TV show The Osbournes on MTV. 

Robert Marcato was on vacation yesterday, courtesy of the Osbournes, and could not be reached for comment, family members said. 

Reagan Marcato moved to Southern California from Mays Landing, N.J., around Christmas 1987 with her young son, Robert, looking for a new start to a life that had been complicated by a teenage pregnancy. 

Marcato made her career in food service and was manager of a popular 1950s-style diner on Sunset Boulevard when she died, the family said. 

Marcato's life revolved around her son, and she successfully battled Los Angeles school administrators to fund a special-education program for Robert, who is dyslexic. Robert Marcato met Osbourne children Aimee and Kelly in school and became close friends with them and the rest of the wacky rock clan.

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