The 5 spoke 19 inch SSV wheels will definitely not look too big on your black Omega.
I would not go with those half baked dark grey variety of SSV rims though. It's personal taste but silver or full black will look way better than those dark grey things, especially on a black car. A few guys have photos of their cars here with the 19 inch SSV wheels painted full black and they look excellent.
For example have a look at the green car on page 5 of this thread (Bradb's car)...
http://forums.justcommodores.com.au/ve-holden-commodore-2006/152423-king-springs-ves-5.html
There are pictures of a few others with the same wheels painted full black but I cannot remember off hand which thread they are in.
2. Bavaria Wheels do their BC10 Concave wheel in 19 inch in matte Black (as well as machine face). These wheels look quite good on VEs in full black. Have a look at the Fawkner Tyres web site for these but I think you will need to go to the Bavaria site to see them in black.
This guy has them on his VE now in black but in 20 inch.
https://forums.justcommodores.com.au/threads/bonners-new-white-e3-clubsport-5.196050
3. Have a look at the VERTINI Magic wheel in Matte Black. These are available in 19 by 8.5 inch and look quite good on VEs. Go to the Ozzy Tyres web site and check them out.
Just a few suggestions for you to consider if you want to go 19 inch full black.