Wallstreets are a mixed breed. They were designed before OpenFirmware was fully defined. As such, they do infact have OpenFirmware, but it's not the same as the New World machines, hence the powerbook is consdered and Old World machine.
It is possible using Quik (the Linux/PPC bootloader for Old World machines) or the "Startup Disk" control panel from OSX PB to have the Wallstreet halt at an OpenFirmware prompt on boot, but you cannot access OF from any of the key commands New World machines use (like Apple+F or is it O+F, I forget)
I used to have a Wallstreet..this thing has Open Firmware just like the other ones (though probably not upgradeable). The disignation of "OldWorld" and "NewWorld" come from whether or not it uses plain old ROM or ROM-in-RAM...AFAIK all PPC machines have OF.
Anyway, to get to OF, you do hold down Command (Apple)-Option-O-F ...you just need to hold down the option key on the right of the spacebar instead of the one on the left. Dunno why it was set up this way, it's a pain in the rear, but that's how it works. Start holding that down before you hear the startup chime, and you should get into it fine.