Yes, Definitely. You can set these parameters manually.

From Docs.
PARALLEL_ADAPTIVE_MULTI_USER =True/False => when set to true, enables an adaptive algorithm designed to improve performance in multiuser environments that use parallel execution. The algorithm automatically reduces the requested degree of parallelism based on the system load at query startup time. The effective degree of parallelism is based on the default degree of parallelism, or the degree from the table or hints, divided by a reduction factor. So set this if you want Oracle to handle parallelism dynamically.

Example: On a 17 CPU machine the default degree of parallelism could be set to 32. If one user issues a parallel query, that user gets a degree of 32, effectively using all of the CPUs and memory in the system. When a second user enters the system, that user gets a degree of 16. As the number of users on the system increases, this algorithm will continue to reduce the degree until the users are running using degree 1, when there are 32 users on the system.

PARALLEL_THREADS_PER_CPU=integer (default value 2) => specifies the default degree of parallelism for the instance and determines the parallel adaptive and load balancing algorithms. The parameter describes the number of parallel execution processes or threads that a CPU can handle during parallel execution, ie the computing power of CPU. 2 is the recomeded value by Oracle.

As long as my understanding DBA have no control over the reduction factor. Oracle is calculating the reduction factor from a "Complicated Algorithm" (metalink) based on the available resources and usage.

Regards

Thomas