Sainsbury's and Asda are two of several companies fined a total of £116m by the OFT for fixing the price of milk, cheese and butter products during 2002 and 2003.
Safeway, now owned by Morrissons, and dairy firms Dairy Crest, The Cheese Company and Wiseman were also punished.
In a statement, the OFT said the parties involved had admitted involvement in anti-competitive practices and had agreed to pay individual penalties. The supermarkets and dairy firms have the option of receiving a lesser penalty if they continue to cooperate with the OFT's ongoing investigation.
In the meantime, the OFT said it will pursue its case against Morrissons, Tesco and dairy firm Lactalis McLelland, having begun its investigation in September.
Consumer group Which? said consumers would be asking themselves what next after milk
Pula Houghton, campaigns pPolicy mManager at Which?, said: "It is shocking that consumers have been ripped off by businesses they trust. Consumers will be asking - if it is milk now, what next?
"If the supermarkets want to restore the consumer confidence that will no doubt be damaged by their admissions, they should publicly commit to never doing this again and find creative ways to proactively reimburse their customers."
She added that while the group was pleased that the firms involved have admitted price fixing and been fined, the fines will not repay consumers who have paid over the odds for basic provisions.
"The Office of Fair Trading must continue to press for the strongest possible punishments to stamp out these practices," she said.
Separately, Tesco's chief executive Sir Terence Leahy has been branded the "godfather of British binge drinking" by John Grogan, Labour MP for Selby.
Grogan, who was speaking in the House of Commons yesterday, said: "Beer is being sold below the cost of water. It is not baked beans we are talking about here, there has to be a totally different attitude.
"I would call on the supermarkets to abandon the practice of below cost selling and also to work out a code of practice against irresponsible promotions, similar to the one the pub industry operates under."
Tesco has since hit back at Grogan's remarks, labelling them as "offensive and inaccurate".
A spokesman for Tesco said: "[The remarks] do nothing to inform the serious social debate on binge drinking. Alcohol abuse is a serious concern to us all and we are playing a lead role in the efforts to tackle it.
"The vast majority of alcohol bought up at Tesco forms part of the weekly family shop. We also know people tend to stock up on beers and wines for home use during promotional periods and consequently buy less at other times."