O’Connor will take over in January from Gordon Bell, who is leaving after 14 years.
O’Connor was previously chief executive of the Olympic Lottery Distributor and, before that, of the Millennium Commission.
He spent his early career at the Department of Health, HM Treasury and the Cabinet Office, before becoming director of the campaign organisation the Coronary Prevention Group.
He who was awarded a CBE for public service in 2000.
"I am honoured and excited to be joining an organisation that is doing such excellent work in tackling problem debt, which causes so much damage to individuals, families and society," he said.
A spokesman for the charity said that under Bell’s direction the organisation had grown from 65 staff to 1,000.
"Having taken the organisation to its current position, he feels it’s more appropriate for someone else to lead the charity as it becomes a more externally focused organisation, representing the interests of indebted consumers to government and in the public sphere," the spokesman said.
In a statement, Bell said: "I am proud to have built StepChange into the largest specialist debt advice charity operating across the UK, helping half a million people a year and employing more than 1,000 people. I leave it in good hands and I wish Mike every success in the future."
The charity recently went through a strategic review that led to it changing its name from the Consumer Credit Counselling Service to StepChange Debt Charity.