".ost" files are not the same as ".pst" files. DLP does not scan ".ost" files.

OST file is a local replica of user's Exchange mailbox. Unlike PST files, OST files are tied to a specific user profile and mailbox. Hence, it is not possible to open and read contents of standalone OST files.

As the contents of OST are a copy of the contents on Exchange server and are synchronized upon connection with the server, it may be sufficient to just use the Exchange scanner to scan mailboxes rather than scanning OST files. Essentially, scanning of OST files cannot be done in the context of a file system scan. It has to be profile based.