I have today received Bertrand Stevenson's NI file. Although it's still not clear why he was awarded the OBE there are a couple of possible clues.
His particular forte was counter-intelligence work, he spoke good German and French and had some knowledge of Dutch and modern Greek reading/writing. In a Foreign Office "Report on the activities of the Commission for the Government etc of Upper Silesia" dated c1923 he is referred to as Controller of Railways at Kattowitz (German spelling of Katowice, Poland) and that his efforts in "the delicate situation at Gleiwitz turned the scale". I don't know what this refers to, possibly the plebiscite in 1919 (Wikipedia link below) as the only mention I've found by googling is in relation to Auschwitz in WW2.
http://en.wikipedia.org/wiki/Upper_Silesia_plebisciteIn a letter he wrote to the War Office in 1940 offering his services (apparently declined) he says he was a General Staff Officer both overseas and on Imperial General Staff [to the Rhine Army] and that in WWI he had devised a "novel and revolutionary method of enciphering messages which is rapid and simple and eliminates entirely the possibility of errors in transmission" and "By the application of technical devices at present used for other purposes....devised a simple scheme.........the use of which would render interception totally impossible".
So maybe it was this encryption method that earned him his OBE?