Sorry, tracking is only a quarter of your problem, you need camber, castor and toe setup as well on on an Omega. As Chris says, Camber is the main issue on these cars and there a two factors that affect this, worn wishbone bushes and GM's incredibly wide factory spec which at best is barely acceptable when the car left the factory.
There are a known set of parameters that are known to work well on these cars that have been developed by Wheelsinmotion.
NB. Any place that sells purely tracking needs to be asked 'what is your reference point that you are tracking too and how do you know it's static and correct'.