First, reset the bios. Second, make sure that the USB is set up in the BIOS. Usually there's an option like "USB Mouse Controlled by: " and the choices are OS or BIOS. Choose BIOS or the usb mouse won't be available until the OS loads. Once everything is working again, then work back up to your OC state. But IMO if something doesn't work on an overclock, then you shouldn't overclock, as the extra strain is obviously affecting something in the system.
Have you tried the keyboard and/or the mouse with an usb->PS/2 adapter?
I used to have a LED keyboard which was USB, but caused all sorts of problems when used with the USB, but I then tried to use it with an USB->PS/2 adapter and worked fine until i changed it for an Saitek Eclipse.
Also i've read that sometimes OC'ing can cause clock problems as jap0nes mentionned. GL with that.