If you rotate your tyres, they wil wear more e evenly,and you wont suffer from as much stagger.
Front to rear, yes (Although I think Chris has a different size on the rear) but not sure I'd rotate left to right. Most tyres for an Omega are directional, so this wouldn't be possible anyway.
As Kev says.
Fronts to rear no problem. Costco insisted they would only fit new tyres to the rear of a rear wheel drive car.
"Off you go then" says I. "See how far you get"

(Rears are 9j)
I know the correct thing to do is fit new to the rear. But I would never ordinarily consider swapping side to side, or rear to front, without expecting some odd feeling through the steering due to pre existing wear playing games (tram lining). Although if new(er) tyre was in better nick then maybe.
Luckily the only option on my particular car is to swap side to side, as the rears are bigger. So it was easily recoverable. After a while

got there in the end.
