roofing sheathing, what kind?

I'm going to have to replace some sections of roof sheathing as part of a re-roof project. What is there now is 1/2" plywood. I guess it's been OK because it's lasted almost 30 years. The part that is a problem was due to not being vented correctly, which I'm going to fix.
I'm going to get quotes and want to make sure everything is spec'd. What is typically used now for roof sheathing? Is OSB ok? is at as good or better than plywood? Oddly looking at Lowes and HD, it doesn't look like they have sheathing in 1/2". Closest is 15/32. I guess maybe lumber yards have true 1/2"? Any code issues anyone might be aware of that are involved?
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