The child, David Banda, was taken to London last week after Malawi's High Court granted Madonna and her husband, director Guy Ritchie, an interim adoption order.
Madonna said David was healthy, despite having overcome malaria and tuberculosis. The interview was taped Tuesday via satellite from London.
"David is amazing," the 48-year-old pop star said. "What really surprises me is how great my children are with him and how he's transitioned so easily from living in Africa in an orphanage to living in our house."
David's father, Yohane Banda, has said he didn't understand the adoption meant he would give up custody of his son "for good." But in an interview posted Tuesday on Time magazine's Web site, Banda said he will not contest the adoption.
"I don't want my child, who is already gone, to come back," he said. "I will be killing his future if I accept that."
Banda has said he is too poor to raise David. Madonna said she met Banda, who thanked her for giving his son a new life. "I sat in that room, I looked into that man's eyes," she said.
The interview with Winfrey was the first time Madonna had spoken on TV about the adoption, which has been challenged by human rights groups that allege the singer used her fame and fortune to flout Malawai's adoption laws.
Madonna said she was worried the firestorm of publicity that surrounded the adoption may dissuade other parents from adopting children from Africa.
"I wanted to go into a Third World country — I wasn't sure where — and give a life to a child who might not otherwise have had one," she said. "I'm disappointed because more than anything it discourages other people from doing the same thing. I feel the media is doing a great disservice to all the orphans of Africa, period, not just the orphans of Malawi."
Winfrey, whose talk show is based in Chicago, commended Madonna for the adoption.
"I have to say, Madonna, that's a brave thing that you did," Winfrey said. "This audience, I know, applauds you for it."
David has spent most of his life in an orphanage. His mother died shortly after childbirth.
Madonna and Ritchie traveled to the southeast African country on Oct. 4. They spent eight days visiting orphanages the singer is funding through her charity.
Madonna and Ritchie were awarded temporary custody of the boy earlier this month. The singer has two children — daughter Lourdes, 9, and son Rocco, 6.
According to viewers who watched the taping of the interview, Madonna told Winfrey she was surprised by the firestorm surrounding her efforts to adopt Banda — and she blamed the media for it.
"She understands, on the one hand she's a celebrity and accepts controversy, but she's trying to do the right thing," said an audience member who watched the taping of the interview from Winfrey's Chicago studio.
Madonna told the "Oprah" audience that she and Ritchie had been thinking about adoption for more than two years, said an audience member from Deerfield, Ill.
Jane Aronson, of the Worldwide Orphans Foundation, told CBS News that the controversy that has swirled around Madonna's adoption was inevitable, given the stark contrast between extreme wealth and poverty involved in the story.
But Aronson also said "there's ample research to show that children living in inter-racial families are quite successful."
"I would say to you that what's important is that children have families, at all cost," she added.