We have someone with Outlook 2010 from an old system that is no longer is use.
Outlook 2010 still works because it's using an OST file to see the emails.
How do I convert the Outlook 2010 OST file in to a PST file and use on our New O365 Outlook 2013?

If the conversion fails you can do this:
Make a new PST file within the outlook 2010 profile, open it in outlook 2010. Drag the email folders from the OST account over into the PST file. Now you can import or just simply open that PST file in outlook 2013 and access the mail items.

To create PST File:
- On the Home tab, in the New group, click New Items, point to More Items, and then click Outlook Data File.
- In the Create or Open Outlook Data File dialog box, in the File name box, type the name as you want it to appear in the Outlook Navigation Pane, and then click OK.

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