is a good practice to create an index on the oid?

I am using the oid of the table as the main key and I've found that is
not indexed (maybe because I have declared another primary key in the table)

it is a good practice to create an index like this on the oid of a table?
CREATE INDEX idoid annuncio400 USING btree (oid);


does it work as a normal index?

Thank you
Edoardo