MS Excel keeps changing my LO calc files to excel

How can I stop Microsoft from changing all my Libre Office .ods Calc spreadsheets to Excel ? This seems to have started happening as soon as I started using Office 365. Last night I carefully saved my spreadsheet in .ods, and this morning it was again converted to an excel format.

Are you sure that is converted or is Excel just insisting on opening your .ods files (file name extension .ods still associated to application Excel…)?

This question is about Excel and thus belongs to a forum about Microsoft Excel.