row oids as "foreign keys" in other tables ?

I'm relatively new to postgres and I've had a couple of questions for a
while now.  This post made me worry about them again:

> 2. the server currently doesn't "reuse" deleted rows, but just keeps
>    appending them to the end.  running a straight VACUUM will perform a
>    de-fragmentation by essentially re-writing the database and then
>    performing equivalent to an 'ftruncate()' at the end to shrink the
>    table size back down again.  The only time you should have to do a full
>    VACUUM is after a massive amount of DELETEs to a table...and,
>    hopefully, the requirement for that will decrease over time too, as
>    there has been talk about adding in functionality to reuse delete
>    rows..

I started to make a database and I wanted to simulate foreign keys
somehow.  So I decided to simply insert the oid of a row in one table into
the "foreign key" column in another table.

For example,

create table concert (
    day_of_show date,
    venue text);

create table song (
    song_name text,
    author_fname text,
    author_lname text);

create table concert_song (
    concert_oid oid,
    song_oid oid);

Then I have a perl script that does my inserts so that whenever I insert a
concert and the songs played, I take the appropriate row oids from concert
and song and insert them into concert_song.

You say that vacuum "re-writes" the database.  Does it alter row oids???
If so, my scheme completely corrupts my database whenever I do a vacuum,
since in concert and song the row oids would change, but my inserted
values would remain the same in concert_song, right?

If vacuum does not alter row oids, then I have another question.  How does
postgres re-use oids?  I've seen the numbers grow and grow, but despite
deletes, etc, I have never seen a lower oid get re-used.  How does this
work?

Thanks a bunch,
Matt