The buses are in full blackout markings and have lamp masks fitted, but with unblanked windows - although in most buses interior lights were reduced to one or two blue bulbs during the war, and the fabric or mesh applied to some side windows was more to protect passengers from broken glass than reduce light (other than forward-facing windows, there was no requirement for buses [or any road vehicle] to have toughened or laminated safety glass fitted until well after the war).
I think the light masks show that it is before the first relaxation of Blackout regulations, in September 1944, and it must have been nearly impossible to see to drive a bus in the autumn evenings along those country lanes. (Although, I suppose, most evening services had been withdrawn for the duration).
The EWL registration series (on the COMS 'decker) was issued from April to November 1937, and sadly I don't have a detailed COMS fleet list to tell me any more about it.
How the photo came to be taken is a mystery - although it was from the negatives of enthusiast photographer RHG Simpson, who did do some wartime photography.
There are some of his photos around of COMS buses taken April 1945 which show buses with lamp masks removed, whilst others taken in June 1944 have them still on.
So I would say, June 1944 when RHGS was known to be in the Oxford area, photographing buses!
Ray Simpson is mentioned on this Flikr page, if you are interested. It appears he was born in Oxford, 4th quarter 1925.