The Labour MP quit the post this evening after the posts became public, Sky News reports.
Mr O’Mara had said “fat women don’t deserve respect” while posting on an internet message board.
He also said “fatties” are “deified” but do not “deserve our respect”.
And he launched a disgusting rant about Pop Idol’s Michelle McManus, who he said only won “because she was fat”.
He wrote: “She only won because she was fat. I also dislike the prevailing western tendency to deify fatties.
“There is nothing noble or admirable about glutting on loads of fatty foods and making yourself obese and ill.
“The only fat people out there who deserve our respect are those who are fat not because of a poor diet/lifestyle, but because of a genuine medical condition. As far as I know, Michelle is not amongst that number.”
Today he said he was “deeply ashamed” of his comments, which were posted during the early 2000s.
He said: “I am deeply ashamed of the comments I made online which have emerged today.
“I was wrong to make them; I understand why they are offensive and sincerely apologise for my use of such unacceptable language. I made the comments as a young man, at a particularly difficult time in my life, but that is no excuse.
“I continue to strive to be a better man and work where I can to confront misogyny, which is why I’m so proud to sit on the Women and Equalities Select Committee.
“I will continue to engage with, and crucially learn from, feminist and other equalities groups so as an MP I can do whatever I can to tackle misogyny.”
This is a developing story, more to follow…