I seem to recall omega's wearing mitchelins from the factory and my local dealer even put them on my corsa SRi but I found they didn't grip predictably in the wet, every car I've had since has had its boots replaced by Dunlops.
They seem quieter than michelins or conti's and I've found them consistente in handeling in all weathers, they said the best way to improve a vectra's handeling was to swap michelin nct 5's
Many Omegas left factory with PS2 - a tyre with good grip initially, but not so great after the first couple of mm worn off. PS4 are in a different league. Dunlop do not make a comparable tyre in Omega sizes - the last great Omega tyre from Dunlop is the now very defunct SportMaxx TT.
How a FWD shitbox reacts to tyres is completely different to how a large RWD exec reacts. Additionally, each chassis has things that make a certain tyre react better...
...for example, my old Rover 25 was probably the best handling FWD car I've ever driven, if you put it on Pirelli P6000's. When they became obsolete, I tried all sorts, including the GY NCT5 you mention, all utter shite, understeered like a bastard. The best compromise - and I went through lots of options - was KU38 Kuhmos, which had pretty good levels of grip (enough to balance the car again), but no feedback, so you had no idea at what point they were going to kill you.