A month before I was searching for a free Ost viewer, then I use Aryson OST Viewer. It is a great tool for opening and viewing OST files without Outlook. This tool allows users to save the OST files into different file formats like DOC, HTML, CSV, EML, MSG, MBOX, EMLX, etc. It adds multiple OST files including attachments and enables previewing of each item one by one. You can also save attachments separately in PDF. 
Another amazing tool I’ve found is OST File Opener. Which is also one of the best Outlook OST viewer.