If you go to SharePoint admin > User Profiles > Set up my sites, there is an option there for: My Site Cleanup "When a user's profile has been deleted, that user's My Site will be flagged for deletion after thirty days. To prevent data loss, access to the former user's My Site can be granted to the user's manager or, in the absence of a manager, a secondary My Site owner. This gives the manager or the secondary owner an opportunity to retrieve content from the My Site before it is deleted. Select whether or not ownership of the Site should be transferred to a manager or secondary owner before the site is deleted. Set a secondary owner to receive access in situations in which a user's manager cannot be determined."
Or alternatively in OneDrive admin on storage tab, you can set how long onedrive sites are saved before deletion. I set my a ways out, so you can still access them after they person leaves for awhile. You will have to use powershell to access / grant permission to these but it works fine.