Trying to recover old emails from an OST file that won’t open is frustrating, especially when Outlook isn’t available. The MSOutlookHelp OST to PST Converter lets you convert OST to PST in just a few clicks, making your mailbox accessible once again. It supports exporting OST data not only to PST but also to user-friendly formats like EML, MSG, CSV and MBOX if needed. Plus, the conversion retains your folder structure, so your Inbox, Sent items and custom folders stay organized after the process.

For more: https://www.msoutlookhelp.com/blog/how-to-convert-ost-to-pst-free-without-outlook/