Set up local folders for outlook.office365.com - Microsoft ...
answers.microsoft.com › en-us › outlook_comJan 06, 2016 · On a PC, I'd create a one or more local data files with File>Open>Open Outlook Data File. I'd then copy the data from the Office 365 folders to the local data files and once the copying finished and I was happy the data had been moved, I'd delete the copied information from the Office 365 folders, empty the Deleted Items folder, close Outlook, delete the OST, then restart Outlook and allow it to create a new OST.