Degas,
I agree with Ashereng, there is no relation between ANSI and WOG. See also MSS-SP 25 "Standard Marking Systems for Valves, Fittings, Flanges and Unions", which is one of the few standards explaining the meaning of "WOG".
Quote from Par. 4.1.3. ( in my 1998 ed.):
The Rating Designation for products which do not conform to recognized national product standards may be shown by numbers and letters representing the pressure ratings at maximum and minimum temperatures.
If desired, the rating designation may be shown as the maximum pressure followed by "CWP" and the allowed pressure at the maximum temperature (for example, 2000 CWP 725/925 F). Products intended for ambient room temperature may show the allowable pressure followed by letters CWP or equivalent.
Note (a) to this paragraph explains that "WO", "WOG", "OWG", "GLP", "WWP" are symbols commonly used in the industry equivalent to "CWP".