O'Sullivan was last seen in South Holland, Illinois on March 7, 1974. She had previously lived in that city, but moved with her parents and ten siblings to Homewood, Illinois in 1972. On the day of her disappearance, she visited a family in South Holland; they lived in the 1300 block of 168th Place, next door to her former home. One of her friends stated O'Sullivan walked to the end of his driveway and hitched a ride with a blue Cadillac. She had a history of running away from her family's residence. She was always located or returned voluntarily to her home within a few days, but after her March 1974 disappearance, O'Sullivan has never been seen again. She may have tried to go to Texas after her 1974 disappearance.

O'Sullivan left her purse and makeup behind, which her loved ones say is uncharacteristic of her. One of the people she went to visit the day she vanished had a drug addiction and another had a reputation for being violent. Two of the boys O'Sullivan visited on the day of her disappearance have since died, as have their parents; the third son in the family was only a small child at the time of her disappearance and doesn't remember anything helpful.

For over thirty years after she went missing, O'Sullivan was classified as a runaway. Investigators now believe she may have met with foul play, as otherwise it is unlikely that she would be gone so long without contacting any of her loved ones. Few details are available in her case.