I thought OS means Oversized.

From their website (they changed it so now I can't find anything ). They used to have a full page devoted to the tubing.

OS2 takes the OS concept to a whole new level. Taking advantage of advanced air-hardening technology, OS2 sports a 1 1/4 inch top tube and 1 3/8 inch downtube with wall thicknesses that offer lighter weight than the comparable OS tubesets with improved torsional rigidity.