Oprah guest starred in the famous "coming out" episode of Ellen, playing the therapist who was the first to hear her big secret.
Talking to The Hollywood Reporter 15 years on, Oprah has admitted she had no idea she would receive so much hate mail and abuse from viewers.
"It always turns to race," said the talk show host. "I got all of the, 'N-----, go back to Africa. Who do you think you are?
"I did it because she asked me to do it and I wanted to support her," Oprah explained. "It didn't occur to me that there would be a backlash."
Ellen also received hate mail and death threats and lost her sponsorship deal with the US clothing company JC Penney.
Describing Ellen's mood on set while filming the iconic episode, Oprah said, "She was pretty emotional that day kind of tense and not fully herself.
"It's one thing to be ready to step out, it's another thing to be ready for the thunderous explosion that occurred after she did.
"Being able to be free literally and to express herself in a way that she can be 100 percent truthful with the audience has allowed them to fall in love with her," Oprah said.
"Honest-to-God truth: I don't believe she would have been as successful as she has become had she not come out."