Thank you for posting your question in Microsoft Q&A.
It depends on if you want to keep the synced user in Azure AD or not.
If you want to keep the synced user then you can delete the Active directory.
If you want to delete the user to Azure AD then you can remove the OU out of synced scope and then run sync cycle. Post that you can uninstall AD connect and then delete the Active directory.
Let me know if you have any further questions.
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