Hello @Alex Tsang !
Yes, you can definitely configure AADC to synchronize users in multiple batches by filtering on the OU level. This is actually a common practice when migrating users from on-premises AD to Azure AD.
To achieve this, you can configure OU-based filtering in the Azure AD Connect tool. During the initial configuration, you can specify the "Organizational Unit" option to select the specific OU(s) from which you want to synchronize users to Azure AD.
Once the initial synchronization is complete, you can change the OU filter in the Azure AD Connect tool to include additional OU(s) and run another synchronization cycle to synchronize the newly added users to Azure AD.
This way, you can migrate users in phases or batches, and communicate the password change and other migration details to a smaller group of users at a time.
Kindly mark the answer as Accepted and Upvote in case it helped or post your feedback to help !
Regards