How can I reduce the salt in oversalted potatoes?
Basically dilution is the key, though a rinse and a short soak in fresh water will help a bit. 

You could freeze them in small quantities and use them up in vegetable soup a little at a time. Don't forget that commercial stock has quite a lot of salt in, so reduce that accordingly. 

Some thick stews could tolerate some mashed veg in them, again in fairly small quantities.

By mixing with completely unsalted veg you could make a form of bubble and squeak, but you'd have to mash/mix it more than normal to avoid having locally salty bits.