The 58-year-old actress and television personally said on Sunday's at-home episode of Watch What Happens Live that Cosby, 82, made an inappropriate remark to a producer while appearing as a mystery guest on The Rosie O'Donnell Show.
"I'm going to tell you a story about what happened to a staff member on our show -- very tall, like six-foot tall, larger woman, very great producer," O'Donnell said. "She was assigned to do his mystery guest."
O'Donnell said she knew the identity of the mystery guest "every time" but would act surprised on the show.
"In the room, the green room, the producer goes in. She's talking and saying to Bill Cosby, 'She's not going to know that it's you.' He says, 'All she has to do is put her hand right here' -- and he put his hand near his penis -- 'and she'll know it's me,'" O'Donnell said. "She started to cry and got up and left."
O'Donnell said a male producer spoke to Cosby after the incident.
"Then our producer, who was an African-American man, the executive producer, he went in and had a big discussion about what's appropriate and what's not. And they didn't tell me about it until after the show," she said.
O'Donnell said she was not surprised by the sexual assault allegations that later surfaced against Cosby, who is now serving a three-to 10-year prison sentence after being convicted on three counts of sexual assault. O'Donnell said she previously tried to share her story about Cosby while co-hosting The View.
"On The View, they would not discuss it. Every time I tried to bring it up it became a little fight," she said.
On WWHL, O'Donnell also explained why Leif Garrett was banned from her talk show. She said the singer used drugs in the green room before his appearance on the show.