According to Airfield Focus 43: Upper Heyford, the first aeroplane to land at Upper Heyford (on ground later to become part of the airfield) was a 2 Squadron Farman en route to Montrose in February, 1913.
160 acres of land were compulsory purchased from landowner, Edmund Graves, in 1915, and construction began soon afterwards. Portuguese (!) workers built Camp Road which bisected the airfield, separating the flying field from the domestic area.
“Before the year was out various aircraft had visited and in 1916 pilot training was in progress and continued during the following year.”
Upper Heyford opened officially in January, 1918.
The earliest unit I can place at UH is 94 Squadron (SE5a) which arrived there from Shoreham on 19.8.1918.