The OFT said it is considering "whether arrangements are in progress or in contemplation which, if carried into effect, will result in the creation of a relevant merger situation under the merger provisions of the Enterprise Act 2002".
The OFT will then decide between three options: whether the merger should be allowed to go through; whether it should be allowed if a number of remedies are fulfilled; or whether the matter needs to be referred to the Competition Commission for further investigation.
The OFT said should the sale of GMG Regional Media be found to be a relevant merger situation, it will consider "whether the creation of that situation may be expected to result in a substantial lessening of competition, within any market, or markets in the United Kingdom for goods or services that warrants reference to the Competition Commission for investigation and report".
Those wishing to contribute, including Trinity Mirror and GMG, should send their representations to the OFT by 3 March.
Trinity Mirror agreed the purchase of Guardian Media Group's regional division for £44.8m last week. The deal, which will complete at the end of March, covers GMG's MEN Media titles and its Surrey & Berkshire papers, but excludes its Manchester-based TV station, Channel M, and two newspapers in Woking.
Trinity said it plans to run MEN Media as a separate business, with David Sharrock, currently chief operating officer of GMG Regional Media, as managing director.
S&B Media will be managed as part of TNS, Trinity Mirror's existing publishing business in the South of England.