The investigation is believed to be focused on a list of more than 20 grocery and toiletry products.
The OFT also visited the offices of Procter & Gamble on Thursday and requested information from Unilever on Friday.
A spokesman for P&G, which owns brands such as Pringles and Gillette, told the Daily Telegraph: "Consistent with our principles, we are fully cooperating with [the OFT's] inquiries. We don't have any additional information to offer at this stage but can say that it is P&G's policy to fully abide by the spirit and letter of the law."
A spokesman for Unilever, the company behind Flora and Dove, also confirmed to the newspaper that the OFT had requested information about price changes.
The supermarkets, which are awaiting the final report from the Competition Commission into Britain's grocery sector, have shown signs of impatience with the OFT's latest action. One anonymous executive said they felt the situation was getting "silly" and the raids by the OFT were like a "fishing trip".
Last week, the OFT was embarrassed by having to pay damages of £100,000 to Morrisons after it wrongly claimed the chain had been involved in fixing the price of butter and cheese.