For sake of clarity - there is no such thing as a backup ost file. These files can only be opened in Outlook by the profile/email account that created it. OST's represent the contents of the server the last time Outlook connected to the account. Exception to this are the folders designated as "This computer only" - those folders must be copied or exported to a separate PST file for future access.
Getting back to your original question - your only option is going to find a 3rd party tool that can convert a stand-alone OST file to a PST file. Do a google/boing search for available options. Or if you want you can use Outlook Recovery Toolbox-which I sometimes use. OstToPst convertor | Ost to pst convert tool | How to repair .ost file of Microsoft Exchange?