usually when the truck goes into slo-mode it will still accelerate but just will not rev up to full rpm's. It should still move though.
if your having to wait an hour before it will move or accelerate enough to move?

sounds to me like a tranny pressure issue
run pressure checks on the control valve and see what pressures you are getting at the pump and clutch packs
pay attention to the inching pedal pressure as well.
check the inching spool and make sure its not enguaged, check the lever the inching cable hooks to and make sure its not sticking.
these things shouldnt cause the 'no acceleration' issue but its one thing to rule out.

as for the oil pressure sender light , get a pressure guage and check oil pressure at the sender, see what pressure your getting. as was said it could just be a bad sender.
and yes a bad signal from the oil pressure sender can make the ecm go into slo-mode or in reality it should shut down the engine in the presence of low or no oil pressure.

just some thoughts to ponder while scratching your head