OST files are the local cache for Microsoft Exchange mailbox data, and shouldn’t be backed up. If Outlook isn’t running, they can be backed up, but it’s not useful because data resides on the Exchange server, and even if the file is lost, it’ll be downloaded from the server again. When Outlook is running, Windows will prevent our backup software from reading it by giving out a 0 byte-long file.

See details at Microsoft Support.