You cannot manually open or import an OST file into Outlook like you can with a PST file. Outlook automatically creates and uses the OST file when you configure an account (like Exchange, Outlook.com, Office 365) with Cached Exchange Mode. You need to convert you OST files into PST file to import them into Outlook to open mailbox. For that conversion you can make use of Trustvare OST Converter. This software will surely help you out in this case. Refer: https://www.trustvare.com/ost/