Hi, yes I know what you mean.
Okay, the middle C, I would say, is one of the weaker notes on the oboe along with B. The lower notes will always have a slightly different sound to it.
For higher notes it's important to play them in tune first - playing high C out of tune can make sound nasty (I know from experience). Most of the time I use vibrato in the upper scale to kill the hardness from them (
it's hard to explain, but I find using vibrato makes them easier on the ears. Few tips - use diaphram for support, if you're have problems getting the top notes on demand then put a little more reed in the mouth. Enjoy!
Anyone else please comment, feel free to correct me.