How to Recover and Import OST Files in Outlook 2021
OST is a proprietary file format for emails and it cannot be directly imported into MS Outlook after disconnection. Because OST file got corrupted and Outlook import option only for PST file format. So, first of all, you have to change your OST files into PST format and then import the PST file into Outlook.
Microsoft doesn't provide any way to export OST files to PST format directly. In this case, you have to rely on third-party applicaion. According to my personal experience, you can go for Softaken OST to PST Converter.