Having done this a couple of times, single course shakes can be a challange to get off, as they don't 'peel' off like a double course of asphalt shingles and you almost have to come up from the down side and split them off.
Now, if the wood shake roof was installed correctly, it should be on 1X4's spaced roughly 2" to 3" apart, meaning you have a 2"-3" shake exposure from the inside of the attic. A heavy steel wrecking bar will usually fit nicely between the 1X4's, allowing you to knock the shingles off from the attic....a much quicker method. If you don't have the solid steel bar, a slightly beveled 2X4 will work to knock them off. I would not knock off the 1X4's, but would put the OSB right on top of them, after checking to make sure they are securely held to the rafters.
Be sure to wear a mask, as this is an incredibly dusty and dirty operation.