CREATE OR REPLACE FUNCTION merge_oid(val oid) 
returns bytea as $$
declare merged bytea;
declare arr bytea;
 BEGIN  
   FOR arr IN SELECT data from pg_largeobject WHERE loid = val ORDER BY pageno LOOP
     IF merged IS NULL THEN
       merged := arr;
     ELSE
       merged := merged || arr;
     END IF;
   END LOOP;
  RETURN merged;

END  
$$ LANGUAGE plpgsql;

Convert an oid value into a bytea value.

Convert OID to Bytea

SQL