Resource Manager is a tool that provides the DBA more control over the resource allocation; this circumvents problems with inefficient OS resource allocation. The OS allocates CPU resources based on OS priorities, Oracle processes have the same priority against the OS scheduler, so it doesn't matter if you launch a CPU consuming task, this process may become a CPU hog. As a DBA there is nothing to do from the OS side as it is not advisable to change the process priority of any Oracle process.
Other problems that may be found at the OS level are:
An excessive amount of context switching, resulting in an overhead when a high number of OS processes is found.
Inefficient scheduling from the OS side: It may reschedule an Oracle server process while it holds latches, resulting in a reduction in the latch hit ratio.
Inadequate resource allocation: This happens because from the OS point of view all processes are the same, and processes consuming a high amount of OS resources will...