Generally, if you delete an employee's account, the content in their OneDrive and Outlook is retained for 30 days. During that 30 days, however, you can restore the user's account, and gain access to their content. If you restore the user's account, the OneDrive and Outlook content will remain accessible to you even after 30 days.
See https://learn.microsoft.com/en-us/microsoft-365/admin/add-users/restore-user?view=o365-worldwide
Note: If you have In-Place Hold enabled. An In-Place Hold preserves all mailbox content, including deleted items and original versions of modified items.