Posted by If you’ve got a stack of large I/O operations to perform, you might want to take advantage of the all of the cores on your server. However, if the scripts are already in place, you might not want to be editing them individually to carefully add parallelism. A little known command is that you can set parallelism at session level. For example, we can control the DDL operations as shown below:
SQL> select * from V$PX_SESSION;
no rows selected
SQL> ALTER SESSION FORCE PARALLEL DDL PARALLEL 5;
Session altered.
SQL> create table T as
2 select d.*
3 from dba_Objects d, ( select 1 from dual connect by level <= 20 )
4 /
Table created.
[in another session, whilst the above is running]
SQL> select * from V$PX_SESSION;
SADDR SID SERIAL# QCSID QCSERIAL# QCINST_ID SERVER_GROUP SERVER_SET SERVER# DEGREE REQ_DEGREE CON_ID
---------------- ---------- ---------- ---------- ---------- ---------- ------------ ---------- ---------- ---------- ---------- ----------
000007FFBD785488 252 27078 474 63158 1 1 1 1 5 5 0
000007FFBD9631D8 481 7594 474 63158 1 1 1 2 5 5 0
000007FFBDB38BE8 714 34791 474 63158 1 1 1 3 5 5 0
000007FFBD5A5668 24 50589 474 63158 1 1 1 4 5 5 0
000007FFBD787558 251 46246 474 63158 1 1 1 5 5 5 0
000007FFBD971788 474 63158 474 0
6 rows selected.
and similarly when it comes to indexing the table
SQL> create index IX on T ( object_id, owner );
Index created.
[in another session, whilst the above is running]
SQL> select * from V$PX_SESSION;
SADDR SID SERIAL# QCSID QCSERIAL# QCINST_ID SERVER_GROUP SERVER_SET SERVER# DEGREE REQ_DEGREE CON_ID
---------------- ---------- ---------- ---------- ---------- ---------- ------------ ---------- ---------- ---------- ---------- ----------
000007FFBD785488 252 62878 474 12776 1 1 1 1 5 5 0
000007FFBD9631D8 481 38744 474 12776 1 1 1 2 5 5 0
000007FFBDB38BE8 714 30697 474 12776 1 1 1 3 5 5 0
000007FFBD5A5668 24 22049 474 12776 1 1 1 4 5 5 0
000007FFBD787558 251 57640 474 12776 1 1 1 5 5 5 0
000007FFBD967378 479 34266 474 12776 1 1 2 1 5 5 0
000007FFBDB3CD88 712 42043 474 12776 1 1 2 2 5 5 0
000007FFBD5B9E88 14 32512 474 12776 1 1 2 3 5 5 0
000007FFBD776ED8 259 42473 474 12776 1 1 2 4 5 5 0
000007FFBD95F038 483 37664 474 12776 1 1 2 5 5 5 0
000007FFBD971788 474 12776 474 0
11 rows selected.
Learning is not a spectator sport 
Thanks Connor.
When it comes to parallel DML, the only way to do that prior to 12c is at the session level.
In 12c and above, this can be done at statement level. with ENABLE_PARALLEL_DML hint.
http://raajeshwaran.blogspot.in/2015/03/enabling-parallel-dml-in-12c_21.html