The sequence must be: Install Windows -> Install Office -> Restore activation -> Restart Windows -> Start an Office program for the first time after the clean install. When restoring the activation using OPA-Backup, there are important considerations: It is a legit and clean way to restore your activation. There is a free, third party app called OPA-Backup, which lets you easily back up the activation of Microsoft Office and restore it. This is not convenient as you have to dial several digits on your phone to send the installation ID to Microsoft and then type the activation numbers back on your device. If you try to re-activate again over the web, you must activate over the phone using the automatic system.
Microsoft allows only a single online/internet-based activation of Office within a certain time period.
Note: See this article if you wish to back up Windows activation.
