The groups have offered to 'divest' Capital-owned station Century 106 FM in the East Midlands to address concerns from the OFT regarding local radio advertising in the region.
The move was expected as Century overlaps with GWR-owned Ram FM in Derby, Trent FM in Nottingham and Leicester Sound FM.
Vincent Smith, director of competition enforcement at the OFT, said: 'Except in the East Midlands, the radio stations of Capital and GWR do not strongly overlap at a local level.
'So for national advertisers putting together a radio advertising package, they are largely complementary rather than competing alternatives.
'However, the merger would result in a substantial lessening of competition in the East Midlands. The parties have offered to divest Century 106FM to address the identified adverse competition effects.'
In addition, Ofcom has also published its assessment of the proposed merger and concluded that 'none of the applicable ownership rules would be breached'.
It has, however, determined that changes are required to two of GWR's analogue licences in Bristol and Bath to safeguard local programming.
The merger will bring together GWR's 33 analogue stations, including national broadcaster Classic FM, and Capital Radio's 21 analogue stations, including London's most popular commercial radio station 95.8 Capital FM.
The combined group, which has yet to be named, will reach approximately 18 million listeners, representing 36% of the UK commercial radio audience.