It's not possible to open an OST file directly in Outlook, as OST files are only linked to the specific Outlook profile they were created with. If you need to access data from an old or inaccessible OST file, the most effective method is to use eSoftTools OST to PST Converter Software. This software easily converts your OST file into PST format, which can then be directly imported and opened in Outlook. With eSoftTools OST to PST Converter, you can recover all your mailbox items, including emails, contacts, calendars, attachments, notes, and tasks, while maintaining the original folder structure. 

Its simple interface and advanced scanning technology even repair corrupted OST files and convert them into readable PST format. After conversion, simply import the PST file into Outlook using the "Import/Export" option, and you'll have access to all your mailbox data again. This software is fully compatible with all versions of Outlook.

Read More:- Open an OST file to Outlook