Nice guide Tunnie and more great pics particularly of the jacking procedure!
Not being picky but you have not mentioned the need to warm the oil up to operating temperature before embarking. I guess that most people on here know this already but for completeness...............
Also, the 2.0 engine takes 4.0 Litres including filter change - well it does in my Cavalier (2.0 16V). Thinking about this though, there may be extra capacity in the Omega owing to the oil cooler so 4.5 could be correct.
I would also add the need to wear latex gloves whilst doing this - oil is carconegenic and you do not really want it all over your hands! Plus, it takes ages to remove without Swafega et al!
On the 2.0 and 2.2 engines, you can fill the oil filter completly as it is virtually vertical. V6 is horizontal so a half fill is about all you can do. I always fill the filter with the new oil at the start of the process as the paper element soaks the oil up. Then I fill the filter again just prior to fitment. This works well and results in the oil pressure light switching off virtually immediatly following start up.
Whilst the old oil is draining out and has reached a trickle, I always put a bit of fresh oil in without the sump plug on. This ensures that, as far as possible, the old oil is drained completly.
The new filter only needs to be done up hand tight. I generally wait until it becomes tight and then tighten another quarter of a turn. Similarly, sump plug does not need to be done up tight - I screw it in by hand until tight, and then nip it up with my socket.
Finally, I change my oil every 3K or around 3 - 4 months.
I hope nobody takes offence at any of this - these are just the things that I do when I change oil. If I have, then I apologise.
Worst still I asked for the old parts and they looked to have been fitted just prior to when I purchased the car.