To be a bit more specific, Yes, 'OR' does mean Other Rank, but it does include NCO's (non commisioned officers). These are the junior NCO's and comprise lance corporals and corporals ( troopers, gunner, sapper, bombardier etc. depending on corps)
The term Rank comes from the formation the soldiers formed up in, usually 2 or 3 lines for marching or firing from (in the ball and musket era).
The Officers or commissioned soldiers were in the front, and leading the troops, the OR's then formed up in ranks, while the senior NCO's (Sergenats and Warrant Officers) brought up the rear. This was to make sure the ranks kept in line, didn't desert or run off in the heat of battle and to make sure the troops had enough ammunition. The other reason the Sergants were in the rear was they are the backbone of the organisation of the army, they run the stores, catering, transport and pay ans so were essential to the running of the force and officers are expendable!
I hope that helps,
John (ex-Sgt RAA)