I've lost count of the number of toilets I've reset because of leaking supply lines or wax rings, and have learned a few things. Your mentioning the leak has been fixed implies that the toilet is back in place--if that's the case, it needs to be removed again to enable properly replacing the sheathing under it.
Cutting the original rotted floor sheathing as nealtw suggests is not a good idea, as it only leaves you with less than 3/4" of joist bearing width for the new and old sheathing. Much better to scab on some glued and screwed 2x stock to the joists, working from underneath. Doing so provides twice the bearing area, and will be less likely to damage the edges of new and existing sheathing by having your fasteners too close to the edges of same. I always drill pilot holes at all sheathing attachment screw locations to minimize splitting, and I also make a point of using exterior grade plywood, to slow down the damage rate of inevitable future leaks.