His salary cost the council £1.20 million over the investigation period.
Mr O’ Sullivan was suspended in March 2013 amid claims relating to pay rises given to senior officers, and then placed on special paid leave.
While Mr O’Sullivan had remained on special paid leave, his deputy Nigel Barnett, and head of legal services Daniel Perkins, agreed to pay-outs worth nearly £300,000 between them after criminal charges relating to the matter were dropped in 2015.
At the beginning of October, in a special council meeting, councillors voted to support a report – put together after a lengthy investigation – recommending Mr O’Sullivan’s dismissal earlier that month.
Total salary costs, including the head of legal services and the acting chief executive, have reached £2.4 million.
Other costs incurred during the investigatory process include £1.39 million in legal fees and £298,000 of settlement costs.
As of November 6, the total legal costs still yet to be paid is £34,455.
Plaid Cymru group leader, Cllr Colin Mann, said: “These figures should be seen against a backdrop of a proposed 6.95% hike in council tax from April and further cuts to frontline services such as turning off our street lights.
“Each and every household in the county borough has paid £85 towards the bill.
“Imagine how many better things that money could have been spent on.”
A spokesman for the council said: “These figures have previously been widely reported and details of the costs have also been disclosed in council reports throughout the period of the investigation.
“It is important to stress that the council was obliged to follow a statutory process when undertaking this investigation and the matter has now been concluded.”