Sounds like your Omega needs a good geometry check.
Spend the £30/£40 on 'four wheel allignment and full geometry check', it will show up the problem
regards
richard a
I would think about getting it done if I could find a place near that does it 
Angie x
Ok, chances are, well, most probably actually, that your camber is out. I strongly urge you to find your nearest 4 wheel alignment centre and go there, no matter how far it is from you. From what your saying the car has never been done in your ownership so the settings have been left and could be all over the shop as well as your current issue.
Trouble is your miles away from your nearest set up centre and you may need new shocks, which obviously you should change first (as the bottom 2 bolts give the camber setting) before the set up, or youll have to set up again, a waiste of money.
So, standard operating procedure is check wishbones(done) stearing idler for play, and springs for breakages. Do any other work you you think necessary(shocks in your case but only you can decide if they need doing, imho they dont ever come up and stop dead, there is always a slight bounce or 2) then get the car set up.
Trouble with driving miles for the set up is your camber is out and will wipe miles of tread off the inside edge of your tires getting there. Normally i would suggest checking the gap from shock body to wheel rim prior to any work, then re set the camber to the same amount when the work is done.
But you dont know the original settings prior to your wishbone change so....find something 15 mill diameter wide(e.g. my 11 mill socket is exactly that) and use it as a feeler gauge between shock and the vertical 10 mill rim edge (not the little lip nearest the tyre). That should give you camber settings close enough to get you to your geometry set up without buggering your tyres on the way.
Hth and ive made it clear enough. :-)