@EEight Thank for that. Turns out the problem was the default RPi keyboard has the tilda '~' remapped to some pipe '¬'. Naughty.

osc~ works now.
I'll post up the console commands to fix it for anyone using an RPi with a standard US/AU keyboard.

I just installed Pd-extended on my RPi2's Raspbian Wheezy (I'm new to linux).
Got audio working (yay!).
All the basic objects work (such as messages, math), but for some reason, all the other objects that should be built in (such as osc~) can't be created. It leaves a dotted red border around the object and says "osc~ cannot be created" in the console.

Do these need to be installed separately? Because I thought these were built-in for Pd-Extended.