There are two methods for converting OST files to PST. The first is the manual technique, which has various drawbacks such as data loss and folder hierarchy maintenance. The second option is to use a conversion tool. I recommend eSoftTools OST to PST Converter to you. You can also recover data from lost OST files and fix damaged OST files. The software also comes with a free trial edition that allows users to export the first 25 items per folder.