RAC and single instance as well ...
OS Watcher is a monitoring application, written by Carl Davis from Oracle Center of Expertise.
And I think is wonderful! install in minutes and work straight away without any problems.
Is the kind of application you may hold on your disk-on-key and install on a client site so that after a couple of hours you will be able to check and see if there is an OS related issue, where the bottleneck is located.
OSW gather statistics using OS commands vmstat, iostat, mpstat, netstat and top. It create one file per hour. This make easy to drill down to a problematic time, and compare it to base line information.
OSW provides also a java interface to display the information in graphic format.
Here you can find a quick start guide I did prepare, you can review the full guide on Metalink OS Watcher Guide
This is an example of the graphics you will be able to display using OSW: