"It's the way I look. He can't resist. I love when people say you're fat like you don't know," O'Donnell joked Monday on ABC's "The View." "... It's always the first comment of someone who disagrees with you if you happen to be on the plus side."
Trump has been hurling insults at O'Donnell since she criticized his news conference with Miss USA Tara Conner last month. Trump announced that Conner would be allowed to keep her title, which had been in jeopardy because of underage drinking.
"Boy, did I hit a nerve with that guy," O'Donnell said Monday, calling Trump a "comb-over bunny" for his relentless attacks on her, which include saying that Barbara Walters, creator of "The View," had told him she didn't want the 44-year-old comedian on the show.
Walters said Monday that Trump's remarks were "totally untrue" and expressed a wish that "we could rise above it."
"It's not we, it's him," said O'Donnell. "He's doing every show in America. He's on QVC talking ..."
"The guy, he's obsessed with me, obviously," she said.
Trump, in a statement to The Associated Press, said O'Donnell had gotten it wrong.
"I used the word 'slob,' I used the word 'degenerate' and I used the words 'not very smart.' The word 'fat' played a very small role, if any, in my description of her,"' he said.
On Monday's show, O'Donnell said she went shopping recently and "all of these plus-size women" with "tears in their eyes" approached her with words of support.
"He wounded millions of women by saying that I was fat, you know?"
Trump is the owner of the Miss Universe Organization, which includes Miss USA and Miss Teen USA.
The new season of his NBC reality show, "The Apprentice," premiered Sunday night.