There are a few OST to PST converters available around, most of them commercial products, with some being better than others. We won't list them here, because we haven't tried any. We have ran into a situation where we've had to get the emails from an OST file. The IT support client did not particularly care what format, as long as they were readable. We've used this free, open-source tool that allowed us to extract the emails in rtf and txt format, and extracted the related attachements.
Please note that this tool does not allow you to convert the files to PST.
The tool can be compiled from source here: https://github.com/libyal/libpff or you can download this compiled version.
Example usage: Output all emails in all formats from file.ost
pffexport.exe -f all -m all file.ost
Full details on application usage: