You need to make sure that the OST file can be opened in its original Outlook profile before you can move the data from OST to PST. If so, open Outlook and use the Import/Export Wizard to save your folders in your mailbox as a PST file.
This method might not work if the OST file is by itself or if the original profile is not available, though. In this case, DataVare OST to PST Converter Expert could be used instead of OST-to-PST conversion. You can look at its features and download the trial version to test the conversion process and see how well it works.