I had the same problem when I upgraded my wife's laptop to 11.10 with KDE. I tried the suggestion above and others I had found with no luck. I reinstalled evolution and Oxygen with no luck. .xsession-errors had a series of lines indicating missing icons when I started evolution (Unfortunately I lost the output from .xsession-errors and I can't provide them here.)
I have a similar configuration on my laptop, and evolution was working fine on it, so I did a stare and compare in Synaptic and corrected a couple of minor difference. Still no joy.
Finally I decided to copy the /usr/share/icons/gnome/16x16, 22x22, 24x24, etc. directories from my laptop to hers. This seems to have done the trick. I made sure to rename the old files just in case I need to revert back.
This is not a correct answer to this problem; there is probably a package missing that I have overlooked somehow. But it seems to work and it saved me from another day of tearing my hair out.