Using OU's for Division/Company Management

We are merging a company into our domain. We want to create an OU and allow them full management rights under that OU, so their users, groups, computer accounts, etc. will all reside under this OU as if it were a domain. That part is easy. What I am trying to figure out though, is how can we restrict/prevent their access to all the OU's we have created in our domain outside of this one OU we are creating for them?

Our environment was never structured previously for this type of scenario so we have about 40 different OU's under our root domain container.

Thanks