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Based on my experience, the user profiles are synced from Local AD to SharePoint Online need two steps. SharePoint Online stores the user profile information separately and the values are synced from the Online tenant domain. Firstly user profiles are synced from On-Premises Active Directory to Online Active Directory by DirSync, and then sync to SharePoint Online by SharePoint Online user profile synchronization service. Therefore after deactivating DirSync, your group data will be retained by the SharePoint Online user profile synchronization service.
If you are using Office 365 for Enterprise, you can sync user's attributes from local AD to Office 365 Directory.
For your reference:
Manage SharePoint Online user profiles from the SharePoint admin center
Planning Directory Synchronization
Let me know if there is anything unclear.
Hi Allen and thank you for your quick reply. Can you please also tell me how this compares with profile sync if we hosted our own sharepoint 2013 server onsite?
If you host SharePoint 2013 server in your own environment, the user profiles can be synced from local Active Directory(AD) to the SharePoint server directly, and you can configure the profile synchronization service by using the SharePoint Active Directory Import tool. This tool allows you to map the user attributes in local AD to user profile properties in SharePoint Online.
Here are some useful resources for the profile synchronization:
Manage user profile synchronization in SharePoint Server 2013
Configure profile synchronization by using SharePoint Active Directory Import in SharePoint Server 2013
Blog: SharePoint 2010/2013 user profile synchronization