pgsql oid question

I have started experimenting with an access layer for pgsql and have a
question.  I had someone on this list tell me that the oid values that
come back from the server are tag identifiers for that row/column
combination and are not type indicators.  Yet, when I create multiple
tables/columns each having the same type (int32), the same oid keeps
being returned.

What am I not getting?  Can someone point me to a good overview of the
internals of PostgreSQL so that I can read up on this?  I have scanned
through the source but still do not have a clear picture.

Thanks
Reggie