Office Online supports the following file types.
Supported files for viewing:
Word files: .doc, .dot, .dotx, .docx, .docm, .odt
Powerpoint: .pot, .potm, .pps, .ppsm, .ppt, .pptm, .pptx, ppsx, .odp
Excel .xls, .xlsx, .xlsm, .xlsb, .ods
Supported files for viewing, editing and annotating:
Word: .docx, .docm, .odt
Excel: .xlsx, .xlsm, .xlsb, .ods
Powerpoint: .pptx, ppsx, .odp