PostgreSQL OID is static, but it will not survive dumping and restoring the database, so you should NOT rely on it.
Actually, in PostgreSQL 8.1 and later, configuration parameter default_with_oids is false
by default, which means that unless you use CREATE TABLE
using WITH OIDS
option, OIDs will NOT be present in your table.
So, long story short - OIDs are thing of the past. Do not use it, do not rely on it, forget about it.