OOPs- Just found the previous Article on the BH Forum about Reg Numbers.
OE was introduced from 1919!!!
Part of the article below.
From 1896 all mechanically propelled vehicles weighing over 14 tons had to be registered. After the turn of the century, the development of the petrol engine brought over 5000 vehicles on the road, and a licensing acr was introduced in 1903 for all motor vehicles, the licences being issued locally. The first number A 1 was issued by the London County Council in 1903. The letter B was used for Lancashire registrations, C for Yorkshire west Riding and so on to E (Staffordshire), and O Birmingham.
The Birmingham Motor Express Company introduced six Milnes-Daimler buses in 1904, which were numbered O 264 - 269. By 1907 the succeeding company (Midland Red) had introduced another 20 numbered O 1270 - 1291 (not O 1279, 1282, for some reason), before it decided to give up petrol buses and use horses again. It took until 1912 to try petrol buses again when 13 buses were registered O 8200 - 8212, followed by O 9913 - 9942.
In that year, rather than add another digit to the number, the authorities decided to add a second letter to new registrations, followed by up to four numbers. In Brum OA started in 1913, reaching OA 2549 by the end of year, OA 4600 by the next. Despite World War 1, the numbers grew to OA 7103 by 1916, when OB followed. Then next allocated letters were OE in 1919 and OH in 1920, then OK in 1922. OL in 1923, OM in 1924, ON and OP in 1926, OX in 1927. Then in 1928 came a foreigner, VP, which lasted for a while, after which some of the missing O-series were filled in, OF and OG in 1930, OV in 1931, OJ in 1933 and OC in 1934. Meanwhile OD had gone to Devon, OI to Belfast. OR, OT, OW to Southampton, OS to Wigtown and OY to Croydon.