Gareth Arnold, who was planning to stand as an independent in the seat, said he had secured a job and would no longer be standing.
He said he hoped whoever did win cared about the constituency.
Mr Arnold dramatically resigned by taking over Mr O’Mara’s Twitter account in July as he said he could no longer sit back and watch Mr O’Mara have such “contempt for his constituents” and believed “something big needed to happen”.
Despite the acrimony, Mr O’Mara later described Mr Arnold as the “Noel to his Liam”.
But announcing his candidacy last week Mr Arnold has insisted Mr O’Mara is “not [his] mate” and he tweeted: “If elected as MP for Sheffield Hallam I would want the office to be held accountable with oversight from a community group made up of constituents. This includes a monthly forum to discuss ideas and issues.”
Other candidates who have said they will stand in Sheffield Hallam so far include Laura Gordon (Liberal Democrats), Olivia Blake (Labour), Natalie Thomas (Green), Ian Walker (Conservatives), and Liz Aspden (Independent).