Because oids where assigned to every data row by default, and were only four-bytes in size, they were increasingly seen as unnecessary. In Postgres 7.2 (2002), they were made optional, and in Postgres 8.1 (2005), after much warning, oids were no longer assigned to user tables by default. They are still used by system tables, and can still be added to user tables using the with oids clause during create table. Server parameter default_with_oids controls the default mode for table creation (defaults to "false").
Oids as still used extensively for system table rows, and are used to join system tables, e.g.:
SELECT oid, relname FROM pg_class ORDER BY 1 LIMIT 1; oid | relname -----+----------------------------------- 112 | pg_foreign_data_wrapper_oid_index (1 row)
Only system tables that need oids have them, e.g. pg_class has an oid column, but pg_attribute does not.
Bruce Momjian is a co-founder of the PostgreSQL Global Development Group, and has worked on PostgreSQL since 1996 as a committer and community leader. He is a frequent speaker and Postgres evangelist and travels worldwide appearing at conferences to help educate the community on the business value of Postgres advances and new technology enhancements. He is the author of PostgreSQL: Introduction and Concepts, published by Addison-Wesley.