It's my opinion that the OA was introduced because it doesn't affect pensions, it is focussed on those immedialty on tour and not the whole of the forces (so a
PR friendly high OA rate could be introduced) and can be dropped as an allowance when no longer expedient. If an appropriate rate had been added to the x-factor instead it would have cost a fortune and would likely prove difficult to remove when no longer justified due to its affect on pensions.