O'Neil, who had early signs of dementia, had been missing for nearly a month.
Investigators believe she got lost while taking a trip to the Everett Mall, just a few miles from her home.
The body was recovered Tuesday from a 1987 Chevy Nova, in bushes off of 12th Street NE near Lake Stevens, Everett police spokesman Aaron Snell said.
The body was taken to the Snohomish County Medical Examiner's Office, which will confirm identity and determine cause of death. Investigators do not suspect foul play.
O'Neil's family describes her as a kind, gentle and caring woman who sang for others at senior homes. Earlier this week, they posted a $5,000 reward for tips in the case.
During a training session Tuesday, a county sheriff's search and rescue helicopter was used to look for O'Neil, according to police. The crews spotted her vehicle about 3:15 p.m.
Everett police and the sheriff's office are investigating.
There are about 97,000 people in the state of Washington living with dementia or Alzheimer's disease. About 60 percent of people with Alzheimer's wander away at least once.
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