The trick here is opening the OST at all, since it is typically tied to a specific profile or Exchange Server account settings. If that account is not available, then you may not be able to open the OST.
If the manual technique doesn’t work out, you can try a third party Download OST to PST Converter for your OST files. It will be definitely useful.
You can export Convert your OST file to PST format
As well you can export OST files to Microsoft office365 account directly using OST to PST Converter of Software Imperial.
You will be able to understand completely about OST Converter program here: https://www.experts-exchange.com/articles/29434/Microsoft-OST-to-PST-Converter-Recovery-Software.html
Move your OST emails to Office 365 account downloading OST to Office365 Converter Software