The OFT launched its probe after receiving complaints from customers that prices charged for groceries on delivery were different to those advertised on the websites.
The five online retailers under investigation -- Asda, Sainsbury's, Tesco, Waitrose and Ocado -- have now agreed to provide clearer information about their online pricing.
The OFT said the supermarkets' websites did not make it sufficiently clear that the prices shown were guide prices and what relation they had to the actual charges.
Supermarkets must now provide up-front information about guide price policies and online shoppers are advised to check prices at the point of delivery against those quoted at the time of ordering.
The supermarkets believe that customers are not contractually bound until delivery, and therefore are free to reject deliveries if they are not satisfied with the prices to be charged.
John Fingleton, OFT chief executive, said: "We welcome the supermarkets' improvement to the information available to customers buying groceries online.
"Customers have a right to clear and transparent information upfront when making their buying decisions."
If you have an opinion on this or any other issue raised on Brand Republic, join the debate in the .