If the deal is approved by the OFT, Dale Farm - a subsidiary of United Dairy Farmers - will gain Fane Valley's customer base.
"The liquid milk market in Northern Ireland has become increasingly competitive, with a combination of increased raw milk and processing costs contributing to declining processor margins," said Fane Valley CEO Trevor Lockhart.
Fane Valley has also announced that it is to close its liquid milk processing plant at Newry, which could result in up to 50 redundancies.
Dale Farm supplies dairy products to UK supermarkets through its Rowan Glen and Loseley yoghurt brands.
The OFT will decide whether to make a referral to the Competition Commission by 4 January.