O/SS is not mentioned in the 1961 NHRA rulebook while Factory Experimental does show up in the 1962 NHRA rulebook. There are photos of cars at Indy with class designation of O/SS and O/SA. Since Stock classes in 1961 went from Super/Stock on top and continued with A/S down through K/S, the designations of O/SS was for Optional Stock Stick and O/SA for Optional Stock Automatic.
Here is the class winner in Optional/Stock Automatic....
