My understanding of Ostrom's work is this: the tragedy of the commons doesn't go away, but we can do better. Historically, the solution to the tragedy has been regulation from on high...an authority (government) metes out a law: thou shalt only have one cow in the commons per household. Punishment is issued for violators.

As I understand, Ostrom said local communities can better self-regulate than a distant authority from somewhere far away. Self-regulation, self-rules, self-enforcement by the locals is more optimal. You still can't herd a 100 cows on the commons, but maybe you can have two for your family of four because your neighbor who lives alone doesn't need an entire cow and you've agreed to share some of your milk with the neighbor.