The TV queen began dating the businessman in 1986, but he has been notably absent from her side at public events in recent months, sparking rumours they are no longer together.
But Winfrey, who has had to fend off long-running rumours of a same-sex relationship with her best pal Gayle King, is adamant Graham is her only romantic interest.
And she is in awe of how he has managed to deal with her superstardom throughout the years.
In a TV interview with revered U.S. newswoman Barbara Walters, Winfrey says, "Yes. (He's) the love, the lover, the man, the mate (in my life).
"I cannot say that I know of another man on this planet who could have lived this life with such dignity, such grace and such respect and humility and still hold his own and be his own."
A Barbara Walters Special: Oprah, The Next Chapter will air in America on Thursday (09Dec10).
The TV queen became visibly emotional while discussing King in a taped chat with broadcaster Barbara Walters, admitting she has relied on her pal for daily support for more than 30 years.
Winfrey says, "She's the mother I never had. The sister everybody would want. The friend everybody deserves. I don't know a better person...
"It's making me cry because I'm thinking how much I probably never told her that. We're not like, 'Oh, you, you're my dearest friend... thanks.' The therapy I didn't have and don't need is because of my thousands and thousands of hours on the phone with Gayle. I get to release the day by talking it through. And I just realised she's the friend everybody should have."
Winfrey also dismissed ongoing speculation about a lesbian romance with King, ranting: "I have said we are not gay enough times. I am not lesbian. Not even kind of lesbian. That irritates me because it means somebody must think I'm lying. That's No. 1. No. 2, why would you want to hide it? That is not the way I run my life."
The TV show, A Barbara Walters Special: Oprah, The Next Chapter, will air in the U.S. on Thursday (09Dec10).