Java EE (Java Enterprise Edition) General Discussion

hi,
i see everywhere that almost in every table we keep a seperate column for primary key which is automatically incrementing and unique which helps in identifying different rows.

what is the need of such seperate column when we have oid already there. as i read it is a unique oid for the whole postgres installation(may be similar case for other databases too). then is there any harm in using oid instead of a seperate primary key column...or is there any advantages with making a primary key column
plz give suggestions.