Yes, there is an OID which Postgre assigns to each row.
Yes, there is also an OID type which you can assign to a column.

If you look in a table containing an OID column, you will see a number which was created by pg, which lets it know how to find the large object data. You don't create the number, you just give it the data and it uses the number for itself. My problem is with inserting the data.

Update: Just a few details from various chatterbox discussions:
I'm using OID because that's the postgres implementation of BLOB. I can't use TEXT, which is limited by pagesize (as far as I know, and I have looked).