@Martin Dodd,Thanks for posting in Q&A.
With the limitation of Q&A, we suggest that you ask Microsoft 365 support to get more help.
https://answers.microsoft.com/en-us/msoffice/forum/msoffice_drive-msoffice_unknown-mso_OPiOS
https://answers.microsoft.com/en-us/msoffice/forum/msoffice_drive-msoffice_unknown-mso_OPandr
Thanks for your kind understanding.
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