To backup and restore the OST file, I strictly suggest and endorse you to go for SysInspire OST to PST converter since I had once used this application. I can surely say that you will not regret it once.
Launch the SysInspire OST to PST Converter and add the OST file by clicking on Add Button.
After uploading the OST select the export option by the drop-down list.
Now browse the destination path where you want to save the exported file.
After that, you can successfully move your ost file to pst and even simultaneously do a backup of the OST file using the SysInspire ost recovery software.
More info:- Backup and restore ost file