Brit Artillery fire control markings

Okay, a bit off topic, but related to the stuff that uses ammo.
What is the meaning of the letters O.S. and G.A. in the markings on fire control instruments such as the following?
Clinometer Sight Mk IV…"O.S. 330 G.A.
Clinometer Field Mk VI…"O.S. 327 G.A.
Clino Sight H. A. Mk I…"O.S. 1156 G.A.\