Remove distributor, replace oring, reinstall distributor. Easy.
However, as has been said, make sure that's what's leaking. IIRC, the disty oring is a "splash" seal that isn't responsible for holding in a lot of pressure. Clean the engine so you can check and make sure it's not the valve cover gasket (they're pretty common) or the oil filter elbow (a pain, but doable) before you pull the distributor. Since you didn't mention that you've checked, I want to make sure you do... Pulling the distributor isn't really difficult, but it's damned easy to screw up and leave you scratching your head on a "no start" condition...
5-90