How to open OST file

If you want to open OST file in Outlook PST format then you should try MailsSoftware OST to PST Converter tool. This GUI tool is designed to convert and open OST files into PST file format. The application preserves all emails, contact, calendar, task, and various other items from OST to PST. You can also move OST files into various other file formats like EML, MSG, HTML, etc.
For more info: Open OST file