Mr O'Mara told an employment tribunal in Sheffield how his fragile mental health deteriorated further after his staff left earlier this year.
But the Sheffield Hallam MP said he travelled to London for key recent Commons votes, despite having a doctor's note until December, because he wanted to represent his constituents.
Giving evidence over a video link, Mr O'Mara was accused by Margaret Flude – one of three former members of staff who have taken him to the tribunal – of using illness as an “easy excuse”.
Mrs Flude said to him: “Is it not true that every time there's a situation that you do not wish to do deal with you fall back on claims of ill health?”
But Mr O'Mara replied: “That's ableist and it's disgusting.”
In April a number of Mr O'Mara's staff left prompting him to close his office for a month.
He told the tribunal on Wednesday: "Things did get worse when my staff left."
Discussed his deteriorating mental health over the summer, the MP added: "It's been a gradual process, going downhill.”
Mrs Flude was asking questions at a preliminary hearing to decide whether the independent MP should been given more time to respond to the allegations against him.
She asked him why he was not able to fill-in the relevant tribunal forms but he was able to do a range of other MPs activities, including travel to Westminster on Saturday October 19 and Tuesday October 22 for key Commons votes.
Mr O'Mara said: “They were important votes and I wanted to go to represent my constituents. It doesn't mean I could fill in a form on that day either."
He explained how his autism, cerebral palsy and mental health problems meant he could do some things but not others, and how filling-in forms was a particular problem for him.
Mr O'Mara provided one of the shocks of 2017 general election when he ousted former deputy prime minister Sir Nick Clegg in Sheffield Hallam but his problems began when a series of sexist and homophobic online posts were uncovered which he had made in his early 20s.
His barrister, Naomi Owen, told the tribunal on Wednesday that Mr O'Mara has indicated he will not be standing at the upcoming General Election.
Mrs Flude told the court that the MP's mother wrote to the court in June to say that her son was “destitute and penniless”.
Employment Judge Robert Little agreed to a retrospective time extension for Mr O'Mara to prepare his response. The judge had rejected an earlier application from Ms Owen for the hearing to be held in private.
Ms Owen cited her client's ongoing mental health problems, saying: "This is someone who is really not very well at all."
She said: "He has felt pursued by the press recently.”
No details of the substantive allegations made by Mrs Flude, Dawn Coop and Kevin Gregory-Coyne were aired at the hearing.
All three have made claims with respect to holiday entitlements while Mr Gregory-Coyne and Mrs Flude also allege breach of contract.
Further case management issues were dealt with in private as the hearing continued on Wednesday.