Which Oracle directory takes up the most space and why?
In a long-running, stable Oracle database instance, configured the &quot;recommended&quot; way, it's probably going to be the Fast Recovery Area.
That's because it holds all sorts of weird and wonderful things, including (but not limited to):

Archived Redo Logs
Database backups, of which there may be several
Flashback Database logs

Not that you can do much about it.  Like most things about Oracle, it will take up as much space as it wants/needs to.  Your job in Capacity Planning your database instance(s), is basically to make sure it never runs out!