There are several methods to convert OST file to PST file.
First, Microsoft does not provide an official tool to do the conversion. However, if you can access the contents of the OST file in your Outlook, then you can perform the conversion manually, as follows:
1. Start Outlook. Make sure you can access all the contents in the OST file to be converted.
2. Create a new PST file.
3. Go to the OST file, select and copy all the contents to be converted with your mouse.
4. Go back to the new PST file, then paste all the contents into the PST file
That will do the conversion.
You can also perform the conversion by export the contents of the OST file to a PST file by following the instructions at:
Second, if you can access the Exchange server containing the account data for the OST file, then you can use Export-Mailbox command to export the data into a PST file, similar to convert your OST file into a PST file. Below is the article about doing that:
Third, if you cannot access the OST file via Outlook, neither can you access the Exchange Server, then you can refer to Outlook MVPs' website discussing about this issue:
Forth, there are many third-party OST to PST conversion tools you can try, below is a list of them:
Nearly all these tools provide a free trial so you can try first to see if a tool works before purchase it.