Charles Fair talks about his research into Officer Cadet Battalions during the Great War.

The evolution of the OCBs stemmed from a shortage of officers from late 1915/early 1916. A system of picking those from the 'right schools' would no longer suffice. There was also a problem with what to do with the 'inefficient', a euphemism for 'incompetent'. 

Men of the 20th Officer Cadet Battalion, shown in December 1917 by which time all would have been expected to have two years service prior to commissioning CC BY-SA 2:0 Hampshire and Solent Museums

Men needed to be recruited and trained from the ranks, especially from amongst middle-class white-collar clerical staff.  

They were trained over four months in a 'mini Sandhurst'. Training included PT, bayonet training, drill, and musketry. Then field skills, bombing, gas and map reading, military law and administration, and finally tactical exercises. Also some social training, to handle informal and formal mess nights.

This fascinating podcasts provides the details :