Zapping the PRAM is a hardware operation, and it was not affected by OS X or anything else.

NewWorld Macs do not have PRAM chips; they have 1 MB of NVRAM (nonvolatile RAM) which does the same things and more. PRAM was just a few bytes owned by the System, but NVRAM contains all kinds of boot preferences and the Boot ROM. This includes POST and Open Firmware.

On a NewWorld Mac, zapping causes the NVRAM to be restored from a ROM chip on the motherboard.

The zapping operation is handled by the PMU99. It therefore does not require the main (PowerPC) processor to be in a runnable state at all. Zapping can restore even an NVRAM with corrupt POST code.

Zapping as a result of cmd-opt-P-R is initiated by the Open Firmware operating system. If your NVRAM is so screwed that you can't even boot far enough to zap (most common cause for me: NVRAMRC syntax error), open your Mac up and remove the clock battery as well as all other power sources (plug, battery - don't remove the power source unit!). This battery powers the PMU99, the auxiliary processor. When it turns back on, it will do the zappin' thang.

Another method I've been using recently (don't ask) is to hold down the little power button on the back of my PowerBook. This is the non-invasive Apple-approved way of resetting a braindead Mac, but you'll have too look up specific instruction for you Mac at Apple's site.