If you want to go for free conversion if Microsoft ost to pst then you should go for SysInspire OST to PST Converter tool. The application allows conversion of the single or multiple OST files into many other formats (MSG, EML, HTML etc) apart from the Outlook PST.
The user can easily use this third party tool software and export the whole or selected OST data along with the Emails, Contacts, Calendars, Tasks, and Attachments in PST in the limited steps.
Step 1: Launch the SysInspire OST to PST Converter and add the OST file by clicking on Add Button.
Step 2: After uploading the OST select the export option from the folder list but before that, you have scanned the file.
Step 3: Now browse the destination path where you want to save the exported file.
Now it will simultaneously convert the data from OST to PST.
More info:- OST to PST Converter tool
ATS OST to PST Converter will find plenty of results, but the best tool is the one that does not alter information and is safe to use. The best tool according to my experience which you give a try for free is ATS OST to PST converter. It has also won many awards for a nice performance. All elements of the mailbox can be restored in the original hierarchical structure. A free demo version is also offered to every user for trial you can try it to perform OST file conversion free of cost.
Read More: https://ats-ost-to-pst-converter-toolkit.en.softonic.com/
To convert single as well as multiple files into various formats like PST, MSG, EML, vCal, EMLX, vCard, MBOX, Live Exchange Server and Office 365 you can go for SysOZ OST to PST Converter tool. This tool help you to have access on your lost and permanently deleted data in few minutes without losing data. There is no size limitation you can convert OST file of any size. It supports all the version of MS Outlook 98, 2000, 2002, 2003, 2007, 2010, 2013, 2016 and 2019. The user directly exports their data into cloud-based application Office365 & Live Exchange Server. It gives 30 days money back guarantee.
Read More: https://www.sysoz.com/products/ost-to-pst-converter.html