If you're using only OD you can set up a master OD and have other MOSX servers bound to that master OD (or it's replicas). The same thing should be possible to do by binding your MOSX server(s) to AD and then having clients authenticating towards the server(s) with AD accounts. It is really pesky to have to sets of users. We are at a transfer state from OD to AD and, well, it's a little messy so I definitely prefer having one user directory.
If you have the possibility to set up a test server (virtualised is wonderful with snapshots and everything, could be possible to do on your own desktop/laptop even, VMWare/ESXi only) I would definitely try to bind the server(s) to AD if that's already existing and see if it works as expected before setting up another user directory.