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).