In a statement, the OFT said that “the period for consideration of this notice has been extended by a period of 10 working days”.
It added that the notice would now expire on October 24.
Last month Tesco agreed to acquire 30 petrol stations from Morrisons, although 12 of the sites were subject to pre-emption rights from petrol retailer BP.
Since then, BP has decided to act on its pre-emption rights and keep four of the 12 sites. A further five sites will be transferred to Somerfield which already has a deal with BP.
The remaining three sites will transfer to Tesco, subject to OFT approval.
Meanwhile, the OFT has cleared Sainsbury’s acquisition of nine former Safeway stores from Morrisons.
In a statement, the OFT said that on the information currently available to it, it had decided not to refer the merger to the Competition Commission.
The deal was originally announced in August and gives Sainsbury more than 175,000 sq ft of new space, with an average store size of 20,000 sq ft.
The stores are located in Hammersmith, Morden, Slough, Andover, Eastleigh, Bitterne, Midsomer Norton, Acocks Green and East Kilbride.