True. However index on non-augmented table can of course be used for *joining predicates*. Question was very general, and general answer is: indexes on both tables involved in outer join can be used.

Simple example on traditional SCOTT schema:

select d.dname, e.ename
from dept d, emp e
where d.deptno = e.deptno(+)
and d.deptno = 10;

If you have DEPTNO columns indexed on both tables (and if optimizer finds both indexes usable for the query) they will both be used in execution of this query. Of course index on DEPT will be used on the driving side, but index on EMP.DEPTNO will be used for outer join predicate.