Abbreviation: OS = "Optical Stabilizer Function" (Sigma)

The abbreviations means "Optical Stabilizer Function"

Explanation from Sigma

This function utilizes a built-in mechanism that compensates for camera shake. It dramatically expands photographic possibilities by alleviating camera movement when shooting by hand held camera.

The OS function uses two sensors inside the lens to detect vertical andhorizontal movement of the camera and works by moving an optical imagestabilizing lens group to effectively compensate for camera shake. The OSfunction offers the equivalent of using a shutter speed 4 stops faster makingit suitable for telephoto and low lightphotography. As the stabilizing feature is built intothe lens, it can be designedspecifically to suit the characteristicsof that lens. This provides precisecompensation for camera shakethroughout the entire zoom range.This also enables the correctlystabilized image to appear in theviewfinder aiding autofocus andcomposition.

See examples of effected products

Sigma EX 17-50mm f/2,8 DC OS HSM
Sigma EX 70-200mm f/2,8 APO HSM DG OS
Sigma EX 50-500mm f/4-6,3 DG APO OS HSM

See links