MS Outlook is only compatible with two file formats that are OST and PST. But sometimes due to virus attacks the offline OST files gets corrupted. So there are two options that either the user can repair that OST file or can convert it to PST. All the items of OST file are imported in PST file format. But this is only possible with a third party software.
I have heard about Aryson Import OST to PST software which easily imports all the data like emails, calendars, contacts, etc. of an OST file in PST file format. The best thing about the tool is that it is the cheapest of among all software available in the market. It just costs $49 and has multiple features.