select we.wf_entity_id, decode(object_type_id, 
                                1, select audit_number from ea_audit_general where sys_audit_id=object_id
                                2,'test',
                                object_type_id
                                ) from wf_entity  we
 where
 we.entry_id in
(SELECT entry_id 
FROM os_currentstep
GROUP BY entry_id
HAVING ( COUNT(entry_id) > 1 ))

decode(object_type_id, 
                                1, select audit_number from ea_audit_general where sys_audit_id=object_id
                                2,'test',
                                object_type_id
                                )

select decode('1',select 'test' from dual ,'2','test2')  from dual

SQL> select decode('1',(select 'test' from dual),'2','test2')  from dual;
DECOD
-----
test2

1 row selected.