Zabbix 1.8 and maybe 2.x created the default tables in Zabbix with OID's explicitly enabled.
The OID isn't used, and isn't created with modern Zabbix installations, and OID's should be dropped from the PostgreSQL instances when upgrading.
Removing OID from a Postgres table involves locking the table for reads & Writes, and will re-write the entire table & indexes.
This process can be. very long, especially with a lot of history or trends in the tables. For our system it was several hours, and the database shrunk with almost 60%!