Set valued attributes ?

Does anyone know how to make an attribute a collection of
OIDs?
I haven't found any reference of this object-oriented
feature in Postgres online manuals.

Please, let me know any hints/links or anything that could help me doing the following:

===
Assume that each row of tables is identified by OID.
I want to define an attribute, say (t1.c2), as a collection
of OIDs linking to rows of table t2.

At the same time the attribute (t1.c3) contains a collection
of OIDs linking to rows of table t3.
===

How do I do to make the postgres "navigate through OID
links", to prevent expensive access methods (joins, sequential or
indexed) ?