The OFT asked the European Commission to hand it responsibility for reviewing the UK aspects of the proposed joint venture.
It expects the European Commission to make a decision on whether it can go ahead with its investigation within 65 working days.
At this point, the OFT doesn't want detail what its concerns are, a spokeswoman said.
T-Mobile's owner Deutsche Telekom is currently working with the Ofcom, OFT and the Commission to iron out any concerns, according to Andreas Fuchs, a spokesman at the German operator, who also declined to discuss OFT's concerns.
The companies' goal is still to have the merger approved during the first half of 2010, Fuchs said. The deal was first announced on September 8.
Orange's parent company France Télécom didn't immediately respond to requests for a comment.
The French operator is also involved in a second merger that has attracted the interest of local competition authorities.
In Switzerland, Orange is planning to merge its local subsidiary with mobile operator Sunrise, which is owned by Danish operator TDC.
In December, the Swiss competition commission started an inquiry that could last up to four months into how the merger will affect competition in Switzerland.
Mike Wilson, manager of mobiles and broadband at Moneysupermarket.com said: "One of the most important factors in any merger like this should be the impact on the consumer,".
"The merger between Orange and T-Mobile would create the biggest mobile operator in the UK and reduce competition in the market to four main providers. It is crucial that the deal receives as much scrutiny as possible."