The Northern Ireland-based company acquired the foundering Sussex Mushrooms business on 28 July, securing more than 400 local jobs in the process and allowing the site to continue production.
The move strengthened Monaghan's already strong position in the UK market, adding further supply to Asda and Morrisons to a customer list that already included most leading supermarkets.
It was also praised by local politicians in the Thakeham area for saving what could have been an economic disaster for the region.
However, at the end of last month (August) the trade watchdog launched its investigation into the merger to ensure that the takeover provokes "no competition or trade issues."
The OFT will be looking to determine whether the acquisition constitutes a "relevant merger situation" under the Enterprise Act and whether it creates a "substantial lessening of competition".
Monaghan Mushrooms chief executive Ronnie Wilson confirmed that the OFT was making informal enquiries into its acquisition of Sussex Mushrooms.
He said: "This is standard practice for the OFT and Monaghan continues to co-operate fully."
The investigation is similar to one carried out a year ago into the takeover by Hampshire-based salad giant Vitacress of the herb production side of Humber VHB.