If you need a free OST viewer tool to open an OST file without Microsoft Outlook, a standalone viewer is a practical option. Softaken Free OST Viewer lets you browse OST files independently of Outlook and view emails, contacts, calendars, notes, tasks, and journals. To use it, install the tool, select the OST file from your computer, let it scan the data, and open the required mailbox folders. It is useful when Outlook is unavailable or you need to inspect an old OST file. If you need to convert OST to PST rather than simply view it, an OST to PST converter would be more suitable.