Criminal defence lawyer Ruth Parker led a 10-year battle to free Faruk Orman, who in July 2019 had his conviction for the murder of Victor Peirce quashed in the first successful Lawyer X appeal.
She condemned the decision to shut the office, which was established to bring criminal charges over the use of barrister Nicola Gobbo as a police source during Melbourne's gangland war.
The outcome reflected an astonishing lack of moral judgement, Ms Parker said.
"It also fails to take into account the corruption that occurred in our criminal justice system involving at least one criminal lawyer and a number of police (which) is, as far as I know, unprecedented in the Western world," the Galbally Parker principal lawyer told AAP.
"It is an atrocious example of corruption spanning years and to expend so much money and time investigating only to disband the OSI... is both cowardly and a sanctification of police corruption."
The disbanding means no one will be charged over the scandal despite $128.5 million having been spent on pursuing it.
Special investigator and former High Court justice Geoffrey Nettle last week said continuing his inquiry would be a waste of time because of constant rejections from Director of Public Prosecutions Kerri Judd.
Ms Judd defended the decision not to lay charges, saying her rejection of three evidence briefs was based on the lack of a reasonable prospect of a guilty verdict.
But Centre for Public Integrity director and barrister Geoffrey Watson SC said the director's decision not to prosecute was unsound.
"The reasons given for it are suspicious and probably legally wrong," Mr Watson told AAP.
"The DPP was wrong to take into account her own assessment of the credibility and reliability of witnesses because that is a question for the jury - not a question for the prosecutor or the judge."
Mr Watson said if the DPP was refusing to act, the attorney-general should have issued the indictments on Mr Nettle's recommendations or given the special investigator the power to lay charges.
"You have a former high court judge identifying serious instances of breaches," he said.
"They've disrupted the rule of law, they've perverted a fair trial and they've committed crimes, which it looks as though they will now go untouched."
Former Supreme Court judge Stephen Charles KC also argued Mr Nettle should have been given the power to lay charges.
"The DPP, who act with and on behalf of the police all the time, were put in a very difficult position by being asked to prosecute people who she will be wanting to work with next week," he told 3AW.
But Premier Daniel Andrews said the royal commission expressly rejected the idea that investigators should be given prosecutorial powers.
"Investigators don't make good prosecutors, there needs to be a separation," he told reporters on Wednesday.
"If (Kerri Judd) and a panel of others make a judgement that no conviction is likely probable, then they move onto the next case."
Shadow attorney-general Michael O'Brien accused the government of giving a free pass to all those accused of perverting the course of justice.
Victoria's worst ever legal scandal would end with a whimper because the government did not want to give the OSI the power to authorise charges, he said.
