You are right....7/16 OSB doesn't have near the compression/deflection strength of plywood. You can test this by taking a chunk of OSB, put it between two 2X's spanning16" and hit in the middle of the span with a 22 oz hammer...it'll break. Try that with 1/2" plywood...it won't break.
But your answer really depends on your situration. If the pitch of your roof is over 8/12, your rafters are 16" OC and you don't live with snow loads of any significant size, OSB should be fine. But when you say 5/8" plywood, this suggest a pitch or 4 or even 3 and/or a Northern climate.
Surely you won't go wrong with 5/8" CDX plywood, and if you have the extra few hundred dollars (you don't mention roof size, so this is a guess), plywood is the way I'd go.
Oh, and make sure you install an externally baffled ridge vent and corresponding soffit vents :-)