He accompanied Fawcett, 59, to a chemotherapy treatment on Monday, the magazine reported in its Oct. 23 issue.
"She's so strong," he says. "I love her. I love her all over again."
In a statement released through her publicist last week, Fawcett said she will receive six weeks of "cutting edge, state-of-the-art treatment."
This isn't the first time Fawcett and O'Neal, who ended their 17-year relationship in 1997, have reunited to fight a medical battle. Fawcett helped care for the 65-year-old actor in 2001 when he was diagnosed with chronic myelogenous leukemia.
The couple has a 21-year-old son, Redmond.