When is parallelism not used by optimizer - but defined?

One of the possibilities is to execute workloads in parallel, But now I am looking for input when parallelism will not work. Especially in the relation to the oracle optimizer, when does the optimiser to decide when not to perform a statement in parallel although parallelisme is defined.

I would like to make a "list" of when not and use this in performance cases. All input is useful,