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:
Use pffexport to export items stored in a Personal Folder File (OST, PAB and PST).
Usage: pffexport [ -c codepage ] [ -f format ] [ -l logfile ] [ -m mode ] [ -t target ] [ -dhqvV ] source
source: the source file
-c: codepage of ASCII strings, options: ascii, windows-874, windows-932, windows-936, windows-949, windows-950, windows-1250, windows-1251, windows-1252 (default), windows-1253, windows-1254, windows-1255, windows-1256 windows-1257 or windows-1258
-d: dumps the item values in a separate file: ItemValues.txt
-f: preferred output format, options: all, html, rtf, text (default)
-l: logs information about the exported items
-m: export mode, option: all, debug, items (default), recovered.
'all' exports the (allocated) items, orphan and recovered items. 'debug' exports all the (allocated) items, also those outside the the root folder. 'items' exports the (allocated) items. 'recovered' exports the orphan and recovered items.
-q: quiet shows minimal status information
-t: specify the basename of the target directory to export to (default is the source filename) pffexport will add the following suffixes to the basename: .export, .orphans, .recovered
-v: verbose output to stderr
-V: print version