The decision was made on the grounds the inquiry was "unlikely to lead to a finding of abuse".
Sparked by a complaint from Express, the inquiry has taken over a year and cost Wiseman more than £700,000.
However, Express' position in the Scottish fresh processed milk market, which it entered in 1998 via a stake in Highlands based Claymore Dairies, remains precarious, say analysts.
"Express tried to expand Claymore and aggressively reduced prices to gain market share, but it hasn't worked," said one analyst. "They have no critical mass. They were banking on the OFT to provide them with a niche in the market and it hasn't happened. Claymore is losing £2.6m a year at the operating level. They will probably have to sell it back to the farmers."
Express said Claymore's trading position had improved following a restructuring of the business.
The OFT said it was still pursuing a second line of inquiry into anticompetitive practices prompted by complaints from "three other companies" as well as Express.
Robert Wiseman chairman Alan Wiseman said: "It is pertinent to point out that during its investigation, the OFT did not make us aware of a single complaint from a customer or consumer."