Unfortunately, Users cannot directly convert an OST file into Microsoft Office 365.

Why?

OST files are copies of your mailbox data that you can reach faster when you’re not linked to the Exchange server. They are meant to be used with Outlook and don’t work with Office 365 directly.

Follow this step: you need to take to properly convert your Outlook PST files to Office 365:

Convert OST to PST:

Use a tool to convert from OST to PST.
If you can get to the OST file, move your info to a PST file by hand using Outlook.

after that Move PST files into Office 365.

You can also use Professional WebbyAcad OST to Office 365 Converter Tool 

​Unfortunately, Users cannot directly convert an OST file into Microsoft Office 365. Why? OST files are copies of your mailbox data that you can reach faster when you’re not linked to the Exchange server. They are meant to be used with Outlook and don’t work with Office 365 directly. Follow this step: you need to take to properly convert your Outlook PST files to Office 365: Convert OST to PST: Use a tool to convert from OST to PST.If you can get to the OST file, move your info to a PST file by hand using Outlook. after that Move PST files into Office 365. You can also use Professional WebbyAcad OST to Office 365 Converter Tool     Read More

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