Yes, there is a simple and free way to convert multiple OST files to PST in bulk. You can make use of ScanPST.exe (Inbox Repair Tool). In any case your OST file is not able to convert via this method, then you can make use of BLR OST to PST Converter Tool. It will help you to convert multiple OST files into PST format easily without any problems and make it accessible in any version of Outlook. I used this software for conversion of OST files and the result was up to the mark. It is a Windows-based tool and works smoothly on all its versions without any hassle.