Welcome to OOF
What are the alignment settings?
The steering shouldn't feel overly light, but there's always a very slight dead spot straight ahead... This is a normal side effect of a steering box when compared to a rack, but weird is never a positive description.
Whilst still shit, one brand of cheap tyres will perform unpredictably under certain conditions when compared to another.
If everything is playing free at the back and you haven't changed anything then I don't see how it can be an alignment issue
To properly check the diff/rear subframe mounts, support but DO NOT lift the diff with a jack. Then undo one of the mount carriers. If the frame falls off, then the mount is done. Refit and repeat the other side.
You won't get any noise when these fail, but it will allow the weight at the back of the car to move in ways that you can't predict.
You need better tyres.
The omega isn't particularly tail happy as a rule.
I've lost the rear of a Vectra C estate more times than any rwd car I have owned. Even the S500 which currently has no ABS/Traction Control.
It's one thing to chirp they tyres pulling out of a junction expeditiously
But if you're trying to slide it at every opportunity, then you'll eventually run out of talent. With luck you'll only damage your ego and the car...
I wouldn't consider any of those tyres to be good.
You need better tyres.
The omega isn't particularly tail happy as a rule.
I've lost the rear of a Vectra C estate more times than any rwd car I have owned. Even the S500 which currently has no ABS/Traction Control.
All the Omegas that I had, share the same tendency to oversteer. Ofcourse if you turn the wheels too much in the beginning youīll break the frontend traction (as exceeding the slip angle of the front wheels 
), thatīs for sure in every car. But regarding which profiled tyres Iīm using the rear likes to break first not the nose, and thatīs completely fine to me.
They corrected it and suprise suprise, it was drivable again! 
On damp conditions itīs very slidy, but managable, you can feel rear and also re-gain traction without that horrible "water on ice"- effect. I would say that rear feels now more like a piece of butter on a hot pan, just like Omega should 
I think the GM documentation says you have to replace the bolt. I do recall its a bugger of a bolt to remove, and you will have to find some way of locking the crank, as the bolt is proper tight!
Could be why it was sold...
Helpful comment......
I used this as a daily for the last spring, but I got few issues. The leftside front caliper was slightly binding (also pads were worn unevenly), cam cover was leaking oil AGAIN, some minor mifires during on accleration and floaty idle as well. During last summer I start work-on with along side with my other projects.
Luckily that happened while adjusting them, so I took old parts off and replaced pins and spring on both sides. Then in September this one passed MoT as well 
Right now Iīm driving 325i and waiting better time to fix this, propably I will replace both calipers to rear or then I will overhaul them by adding new gaskets and pistons. Maybe some other minor fixes to the car also, weīll see
