"I had my name branded in the local press, on TV and the radio before I had even been charged," he said. "I did not even know who was making the accusations."
Mr O'Brien was arrested in October 1998 by detectives investigating claims of child abuse spanning 30 years at two schools in Lancashire.
He was jailed at Preston crown court on five counts of indecently assaulting pupils aged 10 and 11, in 1972 and 1998, but the sentence was later quashed.
"I remember the day I was sentenced," he said. "I got into the prison van.
They had Jazz FM on which was running the result of the trial. That was it with the other prisoners. They would not even be handcuffed to me.
"When I got to Preston prison, I was segregated with the other sex offenders."
Investigations began after a former pupil accused Mr O'Brien of sexually assaulting him in the 1970s. He was acquitted but convicted of other counts.
But Mr O'Brien, now 60, served just six weeks of a three-year jail term before his convictions were quashed by the Court of Appeal.
More than three years on, Mr O'Brien is another staunch campaigner for reform and chairman of Fact. He gave evidence to the House of Commons select committee that recommended a change to investigation procedures.
"Like in my case, it is extremely difficult to prove something did not happen."
"Even if an allegation relates to something that happened a long time ago they should be properly investigated and if the person is found guilty they should face the consequences.
"But time puts a great demand on people to look at all the evidence before starting the legal juggernaut rolling. It can became a sort of witch-hunt where many different people are making accusations over a long period with no right of reply from those accused. "Lives can be ruined and all possible care must be taken during the investigation stages."