Hi,
we have done a lot of migration ot this kind. We published some documentation about it:
http://www.linid.org/projects/openldap-manager/wiki/MigrationSunOracle
You have to study the data, the schema, the plugins...
As said by Ludovic, you will not be able to use the built-in synchronization mechanism, but you can use LSC project to synchronize some data.
Clément.