How to use PICO with OLED and MOSFET?

Did you just add:

function cmd(c){cs.reset();dc.reset();spi.write(c­);cs.set();}
function dispoff(){cmd(0xae);}
function dispon(){cmd(0xaf);}

If it doesn't work, my guess is because you haven't defined cs, dc, or spi variables?

It might actually be you don't have a cs line, in which case you can reduce the whole command to spi.write(0xae,dc);