SQL> create cluster dept_emp(deptno number);

Cluster created.

SQL> drop table emp;

Table dropped.

SQL> drop table dept;

Table dropped.

SQL> create table emp(empno number,deptno number) cluster dept_emp(deptno);

Table created.

SQL> create table dept(deptno number) cluster dept_emp(deptno);

Table created.

SQL> create index dept_emp_idx on cluster dept_emp;

Index created.

SQL>  select * from emp,dept where emp.deptno=dept.deptno;

no rows selected

Execution Plan
----------------------------------------------------------
   0      SELECT STATEMENT Optimizer=CHOOSE
   1    0   NESTED LOOPS
   2    1     TABLE ACCESS (FULL) OF 'DEPT'
   3    1     TABLE ACCESS (CLUSTER) OF 'EMP'

-- thats a cluster join

Never used hash clusters.

SQL> create table t1(x int);

Table created.

SQL> create table t2(x int);

Table created.

SQL> select * from t1,t2 where t1.x=t2.x;

no rows selected

Execution Plan
----------------------------------------------------------
   0      SELECT STATEMENT Optimizer=CHOOSE
   1    0   MERGE JOIN
   2    1     SORT (JOIN)
   3    2       TABLE ACCESS (FULL) OF 'T2'
   4    1     SORT (JOIN)
   5    4       TABLE ACCESS (FULL) OF 'T1'

-- thats sort merge join. A sort on the individual tables followed by a merge.

You could use hints to get a specific join, if you think its going to improve the performance. 

For eg,
SQL> select /*+ USE_MERGE (t1 t2) */ * from t1,t2 where t1.x=t2.x;

no rows selected

Execution Plan
----------------------------------------------------------
   0      SELECT STATEMENT Optimizer=CHOOSE (Cost=8 Card=82 Bytes=2132
          )

   1    0   MERGE JOIN (Cost=8 Card=82 Bytes=2132)
   2    1     SORT (JOIN) (Cost=4 Card=82 Bytes=1066)
   3    2       TABLE ACCESS (FULL) OF 'T1' (Cost=2 Card=82 Bytes=1066
          )

   4    1     SORT (JOIN) (Cost=4 Card=82 Bytes=1066)
   5    4       TABLE ACCESS (FULL) OF 'T2' (Cost=2 Card=82 Bytes=1066
          )