The money is to be repaid because the banks and building societies concerned did not provide customers with some of the post-contractual information about their credit agreements, as required by the Consumer Credit Act 1974.
For example, many failed to provide the required statutory wording in statements and arrears notices.
Under the Act, a lender is not entitled to recover interest or default charges for any period during which it has failed to comply with certain statutory requirements.
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The interest and charges were paid on products such as store cards, credit cards, loans and hire purchase agreements.
The money is to be repaid after the OFT wrote to 50 banks and building societies in November 2013 asking them to confirm that they had fully discharged their obligations to provide post contract information under the Consumer Credit Act.
The OFT took this action in light of previous similar issues relating to the Act by some other banks.
David Fisher, senior OFT director for consumer credit said: "These issues were not deliberate misconduct, but the institutions concerned should have ensured they were complying with the law. The OFT welcomes the proactive steps taken to return money to customers where it was incorrectly charged".