You won't (easily) be able to use OpenDirectory's ldap as a single ldap instance for both OSX and Zimbra. What you really want to do is install OSX Server with OpenDirectory, then install Zimbra with ldap on a port other then 389 (say 1389). You can then configure zimbra to authenticate to an external directory. (see wiki doc
http://wiki.zimbra.com/index.php?tit...Authentication)
You'll still need to provision users in both zimbra and OSX server but zimbra users will authenticate against their OSX server accounts.