My first question is this: Did you ever mod Oblivion?
Still, I don't think the comparison is quite deserved. The games are about as different as WRPGs of the same era get. They're trying to bring forth very different experiences, and unlike, say, comparing FF8 to Legend of Dragoon, The Witcher and Oblivion are so different that making one more like the other in an attempt to "improve" the game would alter the fundamentals of what makes it what it is.
If you want to debate the quality of Oblivion as a game experience, I'd suggest leave The Witcher out of it.
Most importantly, if you are enjoying the Witcher, go have fun with it. You also have Oblivion, so you can enjoy both titles at your leisure. What is the problem?