Going form a JC to a 4-year is fine. Taking 1 class over the summer at a JC to save $$ is OK. Doing badly in an OChem course from a 4-year university and then retaking it at a JC is not.
At the very least, med schools have always stated that one should take the most rigorous option, especially in regards to OChem. This implies taking an upper division courses. JC courses (in everything) are lower division.
Additionally, although my JC OChem course was up to par with the upper division course at my 4-year, adcoms may think otherwise for numerous reasons, including the one I mentioned above. We are all expected to do well in every class, at any school, and adcoms favor those that went the more rigorous route.