The best way to convert OST to PST without data loss, in my opinion, is
Pcinfotools OST to PST Converter. It's best for converting your data without any data loss or corruption because it has protection for data security, including emails, contacts, and calendar. It supports bulk conversion and preview of the mailbox. This can convert orphaned and encrypted files, including calendars and contacts. It provides a free demo version for you to use and understand how it works.