Hello,
Welcome to the Microsoft Community Forum. Please accept our warmest regards and sincerest hope that all is well despite the situation you find yourself in.
I understand your concern. It seems that the OneNote app is no longer supported on your Oppo A3s device. As an alternative, you can access your notes through a web browser by going to onenote.com and signing in with your Microsoft account. This will allow you to view and edit your notes until you find a suitable replacement app for your device.
Hope this helps.
Best Regards,
Fordy B.
Microsoft Community Moderator