Definitely plywood. 4 reasons. 1) Behavior when wet (as mentioned) 2) Ever walked on 1/2" plywood? I don't like the feeling of my roof deck being flexible 3) plywood will provide some rigidity to the roof structure to prevent racking in wind. The same way it does for wall sheathing. 4) The greater mass and thickness will absorb more noise from rain provide slightly greater R value.
Although with that last point. IF you really think venting the attic well is critical to shingle life and that shingles are actually cooled from beneath, then you want really, really think plywood. I however believe nearly all cooling comes form the roof surface from radiation and convective currents or wind.
24" OC is what I have in my pole barn, NOT my house. I would worry about that more than the sheathing.
In terms of OSB vs. Ply, I would only use OSB if it is part of the ZIP (http://www.zipsystem.com/)system which has a plastic coating on it as part of the weatherproofing.
Even with the ZIP system, we used felt paper on the roof and housewrap on the walls.
After over 30 years in this business I do not like using OSB for anything anymore. It (and many other engineered products) simply do not hold up when faced with unapproved conditions which commonly occur anyway. Roofs and walls get over-exposed to moisture and heat sometimes, not by intent but it happens anyway.
And I might add that the engineers who design this stuff almost always lack real-world-in-the-field experience repairing buildings, which to me anyway leaves their products and ideas open to suspicion. I'll bank on my experience and the long-time track record of using only well-proven products and methods which I know will not give problems even when things go wrong. I'm not a luddite, just a skeptic who has seen the real world outside of textbooks and ivory towers where things don't go wrong.
I recommend 1/2" 4 ply cdx and because of the 24" rafter spacing, use plywood clips