In principle the equivalent conductor size of underground cables is much larger of that of the OHL (i.e. a copper cable of 2500 mm2 corresponds with an aluminium overhead conductor of 1500 mm2). Consequently the electricity joule losses are lower. At high circuit loads, OHL losses can be up to 3 times the cable losses due to the smaller conductor size of OHL. Conversely, underground cable losses would be slightly higher for low load currents.
The actual load current on any transmission circuit will vary with both time of day (higher loads during the day, less at night) and season (higher in winter than summer), so calculation of the difference in losses would require data from the transmission company on forecast load and peak demands.