Syncronise only one container between Active Directory

I have a Microsoft Exchange server off-site and a file server on-site (with their own separate AD environment). I want to synchronise AD so that new users and password changes are reflected on both ends automatically. The catch is that the AD on the exchange server end (off-site) has some other OUs that I do not want to sync.

Is it possible to set up a trust between OUs so that only changes/additions made inside the selected OU is synced, rather than syncing the whole of AD?

Would this require a site-to-site VPN connection?

Thanks