Parallel Processing

For safety it is best to set parallel on a session level as needed instead of turning it on at an instance level for all queries. Micro managing PARALLEL DEGREE settings to improve performance of just a handful of queries may allow more queries to run parallel than you expect. Best to set on a session or hint individual queries to allow parallel.
alter session force parallel query parallel 4;
CHarles Hooper has en excellent blog post here that he bills as a lightweight explorarion of parallel query.