I've a feeling it's something like Operational Surplus - a lot of military acronyms made it into common parlance in the war. Just hoping we've an ex-military or older member who knows.
John is right on the button again. OS was operational surplus, (at least it was in 1953, when I was in the forces), and US was as he said, plus MT stood for empty, (when chalked on items) as against Military transport.
I think that should be re-adopted when we have gluts of courgettes or runner beans... offering someone your OS courgettes somehow sounds like they could be something special