tracing the cause of "could not open relation with OID" error

Note that 'reltype' is the OID of the table's rowtype- the OID of the table itself is pg_class.oid (which is a system column, so doesn't show up in \d or select * output, you need to select it explicitly).

Hopefully that will solve some mysteries of how the catalogue tables relate to each other! The same pattern is repeated with quite a few other tables using oid as their primary key.

It looks like quite a serious problem, possibly indicating some sort of catalogue corruption? You can modify pg_class et al directly, but obviously there is some risk involved in doing that. I can't think of much generic advice to give here- what to do will vary greatly depending on what you find.

If this appears while executing statements inside SQL function, then change the language from SQL to plpgsql. The cause can be a cached plan. plpgsql function invalidates plan between runs, while sql function seems to skip this step.