I also received a C on the first semester and then B on the second semester of OChem but I aced the dreaded "Laaaahhhhbooooohhhraaaahhhtoooohhhry". I think you should retake it if you can. It will really help.
BUt it looks like you have a Masters in Biology and you are volunteering at the AIDS FOundation.
Scratch that!!! I don't think the ad com will pick on a C in OCHEm with your masters and voluntary work!!!
Just take it easy and work on your dental experience!