Is there a concern with the CPU usage? I'm not following... Bottom line is having a lower CPU usage isn't a necessary goal. Having low usage just for the sake of it will not mean anything. Still, in order to help you understand what you can turn off/disable, you'll have to tell us what is running.
As for the OC question, you can OC w/o aftermarket cooling, but don't expect to OC too far. With OC-ing comes heat. The stock cooler isn't designed to move enough air to be productive with the higher temps associated with OC-ing.
If you know the specs of your case fans, compare them to the fans available at Newegg
Providing the best-needed environment for your system will help you increase the efficiency. Ensuring you manually configure your BIOS settings will help you increase the performance of your system.