To backup Outlook emails OST files you have to use SysInspire OST to PST converter software. You can load multiple OST files in the bulk conversion mode. In the demo version, you can convert 25 files from each folder of the OST file to PST files. Without the installation of Outlook in your system you can use the software. In the preview panel, you can view the files and folders of the OST files and you can select the files and folders from the list to convert into PST files. You can also convert the OST files into other file formats such as EML, EMLX, MSG, MBOX, HTML, PDF, etc.

More Info- Backup Outlook emails OST files