First of all, trace the location of the OST file. Since the OST file is non-accessible, Follow the Given Steps to Open Your OST File into MS Outlook Successfully:-
Download SysInspire Software for Opening the Outlook OST File, and Install it.
The Software is made to function and the desired OST file is selected.
Select the scanning mode to scan the chosen OST file.
Click on the Next option to get details of the OST file with email items.
Choose the save as the PST option, then click on the Next button.
Now, open MS Outlook for opening the respective OST file.
More info:- Search on google SysInspire OST to PST Converter Software