Check the level colour of your auto gearbox fluid. Check the level with the engine running about 2 minutes after start up. If the fluid is red, its ok; if its brown it has been overheating and if its black the box has serious wear problems. Regardless of Vauxhall's recommendation, it may well be worth draining and topping up the fluid and then every 20,000 miles. Make sure you use Dexron 2 or preferably Dexron 3 rather than ATF
How do you check the fluid? I asked at Vauxhall last year and they said it never needs to be changed, which I though was weird but left it as the box seemed to be alright. It has recently, although not very often at all thumped up or down a gear which is always worrying when it happens. But thats not that often.
Just take the transmission dipstick out and look at the fluid on the end. Vauxhall don't recommend change at any particular interval but it is necessary under extreme conditions (towing, hilly terrain, excessive use of performance!). Checking the colour of the oil will give you the clue. I use my Astra to tow a large caravan so I change the transmission fluid every year, 12,000 miles instead of the 40,000 recommended by Vauxhall for front-wheel drive automatics. The Omega auto gearbox is obviously different but the same principles apply. Check the colour of the fluid, if in doubt, change the fluid.