Copy large object field into remote PostgreSQL DB
I assume that the oid in question is the Oid of a large object, and you are wondering why the large object isn't copied when the oid field is copied.

Large objects cannot be copied via postgres_fdw. You'd have to use the large object API (or pg_dump) to move them from one database to the other. If the Oids are already taken on the second database, you'll have to use different ones and update the table accordingly.

Large objects are cumbersome to handle.