OST is just a pointer to the stuff on the exchange server. You can delete that file and it will rebuild next time you open Outlook, there's not data in it. Converting that to a PST is just a matter of moving the files from the server portion of Outlook to a local email file. Given that the operation is proprietary from Microsoft, I don't see an open source solution.
That said, I also don't see any way to convert it to MBOX except to open mac mail, create a folder "on my mac" and move all the contents there. Once done, don't setup the email as an exchange account, configure as a POP account and don't store mail on the server.
Maybe I'm missing the question or the ultimate goal. But the tools you need to accomplish both tasks exist in either Outlook or Mac Mail. If you are looking for an archival solution for born digital records, specifically email, I'm not sure you would want either PST or MBOX since that just spells an emulation nightmare in a few years. Email is new/simple enough for format migration.
Again, my apologies if I am missing the question.
Manager, Digital Library & Programming Services
Yale University Library
From: Code for Libraries [[log in to unmask]] on behalf of Kari R Smith [[log in to unmask]]
Sent: Monday, June 03, 2013 2:38 PM
To: [log in to unmask]
Subject: [CODE4LIB] WANTED: Open source solution converting OST to PST and OST to MBOX
Anyone point me to an open-source (preferably) or a tried and true solution for
1.. extracting just the PST part of an OST file
2. converting OST file to MBOX format
Kari R. Smith, Digital Archivist
MIT Libraries, Institute Archives and Special Collections
617-258-5568 | smithkr (at) mit.edu