Discussing the safest OST to PST converter that also supports multiple formats, you can make use of TrustVare OST to PST Converter for the best results. Using this application will provide you safe OST to PST conversion process across all Windows platforms. I tried this tool on a 9 GB OST file, and it converted that file quickly into PST format, and all emails were safe. You can also go with this tool and make the conversion safely. Refer: https://www.trustvare.com/ost/pst/