Hi!
I don't know to much about OIFS, but first of all you should consider that Zope's filesystem is a virtual file system stored in one single file (coming with usual advantages and disadvantages). As I remember to OIFS, it does not hold everything in one big file.
Oracle IFS stores stuff to an Oracle DB, that's what it was built for ;-) Zope stores stuff where you tell it to store stuff. If you use the ZODB, it stores to the ZODB (which can be a single file, or a BerkleyDB, or whatever implementation of ZODB you are using), if you use it with SQL RDBMS, it might store stuff there, and LocalFS or ExtFile store stuff to the file system ... (It's that easy, or that complicated, as you like it)
Another point at this comparison could be to find out whether Zope loads the whole file system to memory (as I was told) or not.
I am not the expert on this, but AFAIK, Zope holds some index tables in memory and loads objects into memory when needed, with some caching (e.g. an "objectValues" call will load all the objects in a folderish object like "Folder"). That's far from having the whole system in memory ;-) Joachim