Los Angeles County Sheriff's Department spokesman Steve Whitmore says a judge allowed the 24-year-old to leave jail briefly on Saturday to visit the cancer-striken Fawcett.
O'Neal's family paid roughly $1,300 so that deputies could supervise the visit.
O'Neal has been jailed since April 5, when deputies say he was caught with heroin at a jail facility north of Los Angeles. He faces two important court dates this week, when judges could allow him to re-enter rehab.
O'Neal said in court earlier his month that Fawcett, who is battling anal cancer that has spread to her liver, weighed only 86 pounds. Fawcett's representative disputes the claim.