There is free DEMO tool for OST to PST conversion. Basically all applications allow converting some mails for free. If you want to recover full Outlook OST file then you have to purchase the full version of the software.
Read More :- https://in.linkedin.com/company/atom-techsoft-osttopstconverter
https://www.bayt.com/en/specialties/q/401245/atom-techsoft-ost-to-pst-converter/