The politician, who booted Nick Clegg out of his seat last year, threw in the towel following a suspension from the Labour Party over historic comments on social media which fell below the party’s standards.
Among the allegations were claims he called a woman he dated an “ugly b****” and used transphobic slurs.
Mr O’Mara recently said he was a different person and and apologised repeatedly for the tweets that he claimed was the result of being weak and insecure.
He also said the remarks, made before he became an MP, led him to develop anxiety and depression and that he tried three times to commit suicide.
He told ITV: “I was harassed and hounded by the press.
“I was too scared to go home. I was a mess, I was crying. That lasted for weeks.”
He also said he had autism, adding he wanted more people to be open about mental health issues and not feel ashamed at the stigma.
He also said his autism should not exclude him from being an MP and vowed to stand up for disabled people in parliament.
He was reinstated before quitting today.
Tonight, Mr O’Mara hit out at the manner in which the Labour Party handled his suspension.
He said he felt he had been “made unfairly to feel like a criminal”.
In a statement, he added: “This decision has not been taken lightly but has been made following careful reflection of the decision to re-admit me back into the Labour Party following my suspension last October.
“I feel I have not been listened to or been given a fair investigation as I do not believe they considered my supporting evidence or got in touch with my witnesses.
“Furthermore, I am of the opinion that the Labour Party no longer shares my commitment to the true definition of equality and compassion.”
He added: “There is no doubt that I made mistakes as a young man using distasteful language as a clumsy attempt at satire and sarcasm online.
“But that does not mean that is who I am today.
“I am sure that there may be many of us who have done things in our past which we wished we had never done.
“That said, you can’t take it away and I am truly sorry for any offence that I caused.”
Hi lengthy comment finished with: “I might be leaving Labour but I am still at your side.”
He was elected to parliament in the 2017 election, famously ousting Lib Dem leader and Deputy Prime Minister Nick Clegg.
A Labour spokesperson said the party was “disappointed” he had decided to resign, adding: “We wish him well for the future.”