As per my experience, you can download and install the MailsSoftware OST to PST Converter software. The software allows OST to PST conversion without using MS Outlook and MS Exchange Server. It exports OST file data in PST, EML, MSG, MBOX, MHTML, RTF, and HTML format. The software is available in four versions - Free Demo, Personal License, Business License, and Enterprise License. For more information, you can visit the official website of MailsSoftware OST to PST Converter -

For OST to PST Conversion, I would like to suggest SysBud OST to PST Converter software for Outlook 2019, 2016, 2013, 2010, and other editions. The software is able to convert corrupted, inaccessible, orphaned, damaged and offline OST file into healthy PST file format.
It allows exporting all data content of OST files like- Email, contact, calendar, notes, tasks, attachments, etc into Outlook PST file formats. If you want to export multiple OST file then here you can export multiple Exchange OST files into PST file format at the same time.
For more information, please visit here:- Convert OST to PST

Looking for the Best OST to PST converter tool, then you can use the smart OST to PST converter tool by Data Recovery Solutions (DRS). It is the fastest and safest utility tool to Convert OST Files to PST, EML, MSG, AOL, CSV, HTML, Gmail, Hotmail, etc. with great ease. There is no technical expertise required to perform the conversion using the utility.