The OST is the "offline cache" of the user's Exchange mailbox. It should have everything that was in the mailbox, but that depends on the version of Outlook, the last time it connected to the Exchange server and downloaded the offline cache and address book, and whether or not the file has been corrupted in some way. PST files larger than about 4GB can be bad news too, depending on how they were created (and with what version of Outlook), and whether or not they've been converted from ASCII to Unicode if originally created ASCII.
All in all, these files are never meant as backup solutions, and Microsoft has tried killing PSTs for a long time now (since Outlook 2007 there have been group policies to disable PSTs from being created or attached to the client, for example), and storing them anywhere but locally is unsupported (meaning they aren't likely to get backed up at all, ever, if they're used as a primary mail storage location). Again, I know it doesn't help you at this point, but you're probably in a pretty tough spot already.
What was the history of this OST and why do you have it (without the client install and usage that created it)? It's really not of much use once the profile that fronted the OST has been killed.
