Note that the best explanation of fsoids is in its doctest.

NEWS.txt

@@ -10,6 +10,7 @@ uses of specified oids in the entire database (e.g., suppose oid 0x345620
 is missing -- did it ever exist?  if so, when?  who referenced it?  when
 was the last transaction that modified an object that referenced it?
 which objects did it reference?  what kind of object was it?).
+ZODB/test/testfsoids.py is a tutorial doctest.
 
 
 What's new in ZODB3 3.3 ?

src/scripts/fsoids.py

@@ -34,6 +34,8 @@ by transaction, from oldest to newest.
 
 This will not alter the FileStorage, but running against a live FileStorage
 is not recommended (spurious error messages may result).
+
+See testfsoids.py for a tutorial doctest.
 """
 
 import sys