I seem to get ol mixed up with al a lot. There's probably an awesome explanation out there. I think I try to think of it as than, but it doesn't work exactly right in contexts like antaux ol.
Ol does translate to "than", but you can also think of it as more generally just indicating a comparison or difference. PIV lists these uses for it:
- por enkonduki la duan parton de neegaleca komparo -> to introduce the second part of an unequal comparison (e.g. Ŝi estas pli bela ol li)
- por enkonduki diferencon inter du ebloj, ecoj ks -> to introduce difference between two possibilities, qualities, etc. (e.g. Ŝi havas malsaman ĉapon ol li)
- por signi diferencon de tempo -> to signify difference in time (e.g. Ŝi foriris antaŭ ol li alvenis
If #3 gives you trouble, it might help to think of it as a comparison between the two times (when she left vs. when he arrived), one comes earlier than the other.