Flashing a ATmega328 that came from an Arduino to not use an external oscillator
Do I need an oscillator installed just so I can wake it it up and tell it to not use an oscillator?


Yes, yes you do.

Pick up a 8-16MHz crystal (not an oscillator) and two 15pF capacitors and use very, very short wires to connect them to the chip as shown in the "System Clock and Clock Options" section of the '328P datasheet. You will then be able to reprogram the fuses using AVRDUDE.