The increase in prices led to an estimated cost to consumers of around £270m, the OFT said.
It has issued a statement of objections setting out its provisional findings to Asda, Morrisons, Safeway, Sainsbury's and Tesco, as well as Arla, Dairy Crest, Lactalis McLelland, The Cheese Company and Wiseman.
“This is a very serious case,” said OFT executive director Sean Williams. “We believe supermarkets have been colluding to put up the price of dairy products. This kind of collusion on price is a very serious breach of the law.”