ya that's because when u install nitdroid, it installs its own kernel, which overrides or takes over the maemo kernel.....or something like that anyway.

U have to install kernel-power from app manager, so that when u reach the multiboot menu (the menu where u choose to boot maemo or nitdroid when u turn on ur phone at startup), u'll see another option called: "maemo-kernel-power". That's only available after u install "kernel-power" from app manager.

If u run that, then u'll be able to OC again, but running the stock maemo (the option without kernel-power) then u won't be able to OC.