PRIMARY KEYS

Hi;
I'm pretty new in Postgres.
I've created several tables but I didn't care about PRIMARY KEYS in the 
begining. Now I woul like to add them to the tables.
I've read that PostgreSQL adds an automatic column called OID to each table; 
but when I do a SELECT, the OID column doesn't not appear.
Is this OID column a PRIMARY KEY by default? Can I see it?
How can I see what column is the primary key in a table?
Can I select, at any moment a different Prymary Key?
Can I alter the values of a Prymary Key in a Table?

Thanks and  forgive me for these basic questions.

Best wishes
Javier