How to apply an update after an insert or update in POSTGRESQL; I have got a table which has a field lastupdate; I want that field to be set up whenever the row is updated or when it was inserted. I tried this trigger, but It is not working! HELP!!

CREATE OR REPLACE FUNCTION fn_update_profile() 
                             RETURNS TRIGGER AS $update_profile$
    BEGIN
    IF (TG_OP = 'INSERT' OR TG_OP = 'UPDATE' ) THEN
           UPDATE profile SET lastupdate=now() where oid=OLD.oid;
           RETURN NULL;
     ELSEIF (TG_OP = 'DELETE') THEN
       RETURN NULL;
        END IF;
    RETURN NULL; -- result is ignored since this is an AFTER trigger
    END;
$update_profile$ LANGUAGE plpgsql;