We have found 2 software records in our database eligible for .ost to .pst file format conversion.
The obvious choice for ost to pst conversion would be to to use Microsoft Outlook, considering both .ost and .pst files originate from that program.
You can export inbox off-line folder file (.ost) to personal folder file (.pst) in Microsoft Outlook by selecting the File ► Open ► Import ► Export to file ► Outlook data file
If you do not have access to Outlook, you have to rely on other 3rd party conversion or recovery tools that help users with transferring Outlook data.
Mac version of the popular e-mail client and personal information manager from Microsoft
OST
An ost file extension is used for Microsoft Outlook Offline Folders files. The ost files are used to store e-mail messages and other data download from an Microsoft Exchange server. Microsoft Outlook Offline Folders files are very similar to Outlook personal folders files (.pst files).
PST
A pst file extension is used for storage file format of Microsoft Outlook. PST file contains stored emails, calendars, contacts in one single file.