To convert OST to HTML, use MacProTools OST to PST Converter for Mac. Without any issues, this tool exports all OST emails, attachments, calendars, contacts, and notes to PST files. Before beginning the conversion process with this tool, which has many additional capabilities and filters, users can sample the chosen OST files using the Advanced sample option. All Mac OS versions are compatible with this app. You can retrieve the interrupted OST database in addition to that. The user interface of this tool is straightforward but engaging. It offers a free trial version for evaluation.

https://www.macprotools.com/blog/convert-ost-to-html/