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...