When someone leaves the company with a bunch of data left behind in their OneDrive account, the easiest way to access this data without going through the whole sharing exercise is probably to grant others administrator rights to this OneDrive account.
Heading over to the SharePoint admin center and signing in with a Global Admin account.
Click the More features on the left pane and the Open button under the User profiles section.
A classic SharePoint admin page opens up. Click Manage User Profiles under People.
Type the name of the account into the Find profiles box and click the Find button to find the user account. Once found, right-click the account and choose Manage Site Collection Owners.
Now you can add more users to the Administrator’s group and click the OK button once done.
Once it’s all set, that user’s OneDrive folder will be showing up automatically in the newly added administrator’s OneDrive folder.