Is optical density the same thing as index of refraction? Is there a difference?

Optical density is nothing more than a vague colloquial term for index of refraction.  Index of refraction is the one which is actually quantified.  Optical density is just a bit of a figure of speech in order to give a hint as to the cause of index of refraction.

Only index of refraction and angles relate to Snell's law

I don't know that it is a "colloquial" term - as the previous answer says. The Penguin Dictionary of Science defines a material with a higher refractive index as having a higher optical density.

But I think there may be another meaning, from astronomy. Imagine a cloud of hot, glowing gas, which is nevertheless so thin that you can see stars behind it. Then imagine another such cloud, through which it is more difficult or impossible to see the background: I would describe that as more optically dense. That has nothing to do with refractive index; perhaps we should reserve the term for the latter context.

There are two definitions for optical density.  First, it's an alternate name for index of refraction.  Second, it the logarithm of the fraction of light the material absorbs.