Yup, although GM has made significant progress with it's other small car brands by using Opel's platforms, so it's a two way street. SAAB, on the other hand, were even more notorious than Opel for reinventing the wheel, and GM got a lot less back...
I know GM were trying to push Opel into the brand space SAAB used to occupy, but that pretty much hasn't worked - Europe don't see Opel as a premium brand, and Vauxhall is only doing well in Britain because they didn't follow that edict, and quickly returned to competitive pricing.