[JoeyBob]

Why do bases catalyst cause the hydroxyl group to go on the less substituted carbon while acid catalysis does the opposite?

[hollytara]

Kind of odd, right? 

Basic ring opening of epoxides is with OH- or something similar - a strong nucleophile.  So it "obeys" SN2 rules and reacts fastest with the less hindered position.

Acidic ring opening works by protonating the O of the epoxide to make a cation.  The positive charge also delocalizes to the ring carbons - and more of it delocalizes to the more substituted carbon.  So the weak nucleophile (H2O or similar) goes to the carbon with more + charge density - the more substituted carbon.  More like SN1 - make the better cation.

Does that make sense?  There is also a good explanation in the book by Jakob Fredlos.