Make sure your cold temperature light is off before verifying whether or not O/D is engaging or not. The O/D will not engage until the engine has reached the approprate operating temperature. As an example in Winter it could take five minutes or more of driving before the engine reaches that 'magic' temperature and the O/D kicks in. Make sure the 'test' you did was more then just around the block.
Or are you saying the O/D button isn't toggling the the light on the dash?