The answer changes for OCF, if you use the onboard flash as the Flash Commander. I use the Flash Commander in my 60D. In order to have full control of the OCF from the camera, you set the OCF in ETTL II slave mode (A or B, depending how many flashes you are using and what is the desired contribution of the onboard flash...). ETTL is what allows communication with the camera so you can manually change the flash power from the camera.
Now, if you set:
The onboard flash to ETTL, then the flash power will be set by the camera, based on its algorithms. You can set the power ration between onboard flash and OCF. You can use Flash Exposure Compensation from the Q screen.
The onboard flash to Manual, you can set the flash output power manually for groups A and B. You lose the Q screen control.