Hi there!
Are you using Android or iOS, and which versions of OneDrive and Excel are installed? Microsoft supports opening Excel files directly from the OneDrive mobile app, but if Excel opens a file as a text document, it usually means the app doesn’t recognize the file properly.
To fix this, make sure both OneDrive and Excel are updated to the latest version, open the Excel app first and use its Open menu to access the file from OneDrive, verify the file extension is exactly .xlsx, and check the same file on OneDrive web to confirm it opens correctly in Excel Online.
Let me know if that works.
Best,
Arlene