Microsoft doesn’t provide tools to transformation ost to pst, you will need to search on the Internet for this kind of tools. In my knowledge, this OST to PST Conversion tool has good customer review. This tool uses secure algorithm to recover and convert OST files to importable Outlook (.PST) files. Before executing this migration procedure, be sure that Outlook must not be connected with Exchange Server and Outlook installation is required. You should visit microsoft technet forum site for once, may be you will get another useful answer.