"She's just been so supportive with advice, practically speaking, spiritually speaking, emotionally speaking," the actor told PEOPLE at a special event celebrating his new miniseries 'Lawmen: Bass Reeves'
The actor, 47, said the talk show icon, 69, has become "like a mother" to him since the pair first shared the screen in the 2013 film The Butler.
"She's like a mother to me and has been since we did The Butler, where we actually played mother and son," he told PEOPLE on Friday at a special event in Los Angeles celebrating his Paramount+ miniseries, Lawmen: Bass Reeves.
Winfrey hosted the event, and the two friends — who also starred together in 2014's Selma — walked the red carpet arm-in-arm.
"She's just been so supportive with advice, practically speaking, spiritually speaking, emotionally speaking," Oyelowo continued of The Color Purple star, with whom he has continued to collaborate on projects. "She has been with me on this journey, this eight-year journey to getting not only Bass Reeves made, but Selma made. She's one of the reasons that show, that film, exists."
Oyelowo also credits Winfrey with inspiring him to take a stand for himself and the things he believes in. "She taught me what advocacy looks like because she has advocated for me enormously, and that's something I'm now trying to carry into my own life as well," he explained.
In a touching speech at Friday's party, Winfrey had high praise for Oyelowo, describing him as "such a lovely human being."
"[He's] such a spirit-centered, such a giving, gracious human being. And I am so glad that he is receiving the rewards of his labor," she said, referring to his career successes.
Winfrey even revealed the sweet nickname that Oyelowo calls her. "He's the only person outside my circle of daughter girls that is allowed to call me Mam-O because everybody else calling Mama O, no, it doesn't work for me. He's the only person allowed," she told the audience.
In his own speech, Oyelowo again touched on Winfrey's maternal influence in his life. "Over a decade ago, we played mother and son, and you have been that for me ever since," he told Winfrey.
He went on to share how the entertainment mogul supported him after his mother fell seriously ill. "Two months before we started shooting Selma, my mom had a brain aneurysm and she went into a vegetative state, a state in which she stayed for three years before she eventually passed away," he said. "It was the hardest thing I've ever been through."
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Oyelowo said that after the first day of filming ended on Selma, he went home and sat alone, feeling "bereft, just broken" because his mother wasn't able to share in his emotional experience of playing Dr. Martin Luther King Jr. in the film. Then he got a call from Winfrey.
"And she said, 'How are you doing?' And she asked me how I was doing and how the day had gone, and the most precious moment... I'm a grown man. My most precious moment was when she said, 'Time to go to bed,' " Oyelowo recounted, choking up with emotion.
"You've been an incredible mother to me," he told Winfrey. "I remember the first time we had a conversation in New Orleans while we were shooting The Butler, and you talked about how Quincy Jones and Sidney Poitier had been there for you and we had only just met. And you said, 'I see something in you.' And you said, 'I'm going to be what they were for me to you.' "
Winfrey, he explained, made good on her promise. "She saw in me what I didn't see in me yet, and that's why Bass Reeves has come to fruition, because you gave me belief in myself," he added.
In his new series, which he both stars in and executive produces, Oyelowo portrays legendary Wild West lawman Bass Reeves, a former slave who gained his freedom to become the first Black U.S. Marshal west of the Mississippi River. The actor told PEOPLE he spent a lot of time learning to ride horses in preparation for the role.
"I rode horses for over a year in order to be able to convince the audience that I seem like I'm Bass Reeves, because he was extraordinary at that," Oyelowo said. "And yeah, that's a skill I will never let go of. I love horses, I've always loved horses. I wanted to be a cowboy as a kid, and now I have that ability."
Oyelowo said he's proud to be bringing Reeves' story to the screen. "You know, everything on paper suggested this man was worthy of that treatment. But the fact that the audience is embracing it is hugely validating and vindicating," he told PEOPLE.
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