The Daily Mirror reported that the politician was arrested a week ago along with his chief of staff Gareth Arnold - who, it is alleged, was arrested on suspicion of conspiracy to commit fraud.
Officers are said to have raided the MP's constituency office, removing documents and computers for investigation last Friday.
The Daily Mirror said the two men were released on Saturday evening pending further investigation.
Despite national media reports, the arrests have not been confirmed by South Yorkshire Police.
In a statement, the force said: "We neither confirm nor deny the identity of someone who is or is not subject of investigation.”
The Star rang Mr O'Mara and Mr Arnold but neither answered the phone.
We also called at Mr Arnold's house but another man opened the door and said: "No thanks, sorry, bye."
It is understood Mr Arnold contacted police in early July to express concerns about Parliamentary expense claims made by Mr O'Mara.
Mr O’Mara has had a turbulent time in office since ousting Nick Clegg from his seat in the 2017 General Election.
In July, his chief of staff Mr Arnold resigned via a foul-mouthed rant on the MP’s own official Twitter account.
A 20-year-old woman also accused the politician of sexual harassment.
The 37-year-old MP later resigned and said he was ‘not well and in the process of receiving medical help’ and would resign following the MPs’ summer recess, which is due to end on Tuesday, September 3.
Mr O’Mara said: “Please let everyone be assured that I will be tendering my resignation via the official Parliamentary process as soon as term restarts.
“I am not in any fit state to continue and nor would that be appropriate if I was. I reiterate my apology to my constituents, the people of Sheffield and the people of the UK as whole.”
In April, the politician closed his constituency office for a month, there were reports that all his staff were either sacked or walked out, and he stopped holding surgeries or responding to correspondence.
He also also quit the Labour Party after being suspended over historic misogynistic and homophobic comments made online.