The "Selma" star's mum suffered a fatal brain aneurysm four years ago.
Speaking to U.K. chat show host Jonathan Ross, David reveals that before her death, his "The Butler" co-star Oprah visited her in hospital to help his mum when he couldn't be there.
"To be honest, the reason I call her (Oprah) Mummo, this to me is what the heart of this season is and what she represents," he explains on the Christmas Eve, December 24 edition of Jonathan's show. "My mum had a brain aneurysm about four years ago and she ended up passing from it. Oprah knew about this and she knew how much my mum meant to me."
The British actor went on to reveal how he had to frantically warn his dad that the U.S. talk show legend was on her way to see them in London.
"My mum was in hospital with my dad here in London," he adds. "I was on set, I suddenly got a call from Oprah, she said, 'I'm in London, I was on my way to South Africa, and I know your mum is here, right?' She said, 'I want to go and see her.' I told her where my mum was, she went, then called me again from the hospital lobby."
"I called my dad, 'Oprah is coming to see mummy, get in the hospital now'. He was stuck in traffic. She was sat in the hospital lobby on her own. She called me and said, 'Where's your dad?'. I said, 'He's on his way, but he's stuck in traffic.' "
Tearfully recalling how Oprah was there for his family, he continues, "She waited for my dad for an hour and a half and then spent two hours with my mum... That's why she's my Mummo."
The Christmas Eve episode of "The Jonathan Ross Show" will air on U.K. network ITV.