Symptom

You upload an excel spreadsheet to your One Drive for Business folder to enable viewing and editing from Excel Online.  When you click on the file it downloads to your computer instead of opening up automatically in Excel Online.  On closer inspection you may notice that the excel icon is slightly different to the other excel spreadsheets in your account.

Reason

The file has been created in an older excel version as a .XLS file instead of a .XLSX file.

XLS-vs-XLSX

Resolution

Download the file to your computer, open with Excel and save as a XLSX file before uploading back to One Drive for Business.