So it seems this is not as straight forward as importing a PST file. OST files are weird magic, and most of the suggestions I can find talk about buying software to do this. So just incase someone else has the same issue, here is how I solved the problem. This does require access to the mail in the OST file through Outlook, which may not be possible in your situation.
Machine A = Outlook with OST file, old mail
Machine B = Outlook
- Open Outlook on Machine A
- Create a new, empty .pst file (yes pst). Make sure you save the pst file somewhere you can find it later.
- Copy all the mail you want to transfer into the newly created pst file. I suggest COPY and not MOVE because you never know what can happen.
- Close Outlook on Machine A
- Copy the newly created .pst file onto Machine B. You may need to wait for Outlook to completely close the .pst file. Worst case, if Windows complains that the file is still open, reboot the machine.
- Open Outlook on Machine B
- Import the newly created .pst file
Voila. YMMV. I take no responsibility for any loss or damage incurred following these instructions.