what offset are they and what offset does a standard omega have ? RWD and FWD vauxhalls used to have significant offset differences, which would make the handling weird.
Omega offset depends on wheel width... Numbers below are off memory, but confident that they are accurate
iirc, 16" are 6.5J ET38 225/55/16
17" are 7.5J ET35 235/45/17
18" are 8J ET30 235/40/18 (fronts) and 9J ET30 245/35/18 (rears)(Sportstars)/8.5J ET33 245/40/18(Stila)
All the Astra/Vectra stuff is ET4*.
There will be people who have successfully fitted XYZ wheels on an Omega, but what works on one car won't likely work following suspension work or with different geometry/parts. Oh, and using spacers is a dangerous bodge.
Basically, either fit the correct wheels or aftermarket with the correct offset for the rim width.