this area is very difficult to pin down imho. If your car doesnt track straight there are many causes. You can check the chassis and steering until your blue in the face, change seemingly faulty parts all day long and get no where.
Why? Tyres. Some seem to work on the omega, the expensive ones it seems, others dont. The owners manual even states not all tyres are suitable for these cars, something i was rather shocked to read very recently.
However if you've noticed a sudden deterioration in the car behaviour, and the car has been set up correctly, esp during the life of a set of tyres then something has failed, or worn.... IME anyway.
So question for the op. What made you jack the car and look? You mention crab or tramline behaviour. When did this start? Has it always been there or has it shown itself recently? If recently, which possibly sounds more likely? it would suggest play somewhere.
Wheel bearings can have play but not fail, i nipped mine up to take out the play, on the nod of "the master". Wheel still spins sweet as a nut. You will need a big breaker bar though and dont over do it. Better to have a fraction of play than over tighten. The Uk MOT allows a small amount.
Steering idler(grab it and wiggle up and down) and track rod ends tend to show wear when examining those individual parts. So should be fairly easy to spot.
Steering box seems to have a bit of play when the engine is off, only to be taken up when engine running and the power steering runs taking out the slack. If there is play with engine running, which can be felt by feeling the front wheel and moving the steering wheel at 90degrees off straight, through the open window, then it can be adjusted out. But you need to be very carefull with this procedure as its easy to over do it and screw up.
If, after all that lot, you dont find the problem, then would think your problem may well be wishbone bush related. If its tyres, you probably would notice soon after fitting.