Here in the Blue Ridge mountains of Virginia, white oak (not red oak) is noted for its rot resistance. Even better is black locust, which old-timers say lasts two years longer than stone.
That aside, the best protection for any timber is to detail the roof and siding so that water never gets to the timber. Would it be possible to raise you shed 12" off the ground on stone pillars and extend the (reasonably easily replacable) siding an inch or so below the sills?
