OID Behavior

Is there a summary anywhere describing 
OID behavior?  Specificially, what you
can do with the system generated column
and what you cannot do?

For instance, 

a) You cannot update it.
b) You can index it.

From what I remember, someone said that
you could export data with OID and
then import it.  

When is the above true?  When the import
and export are performed on the same 
database?  If you change databases, etc?

In particular, knowing how it is used
internally would help a great deal.

Thank you!

Clark Evans