OST or Offline Storage Files represent user's mailbox data in offline mode on local machine upon registration with Exchange Server using Microsoft Outlook. It is automatically created on the first use of Microsoft Outlook upon connectivity with server. Once the file is created, the data is synchronized with the email server so that it is available offline as well in case of disconnectivity from email server.
Files with OTT extension represent template documents generated by applications in compliance with the OASIS' OpenDocument standard format. These are created with word processor applications such as free OpenOffice Writer and can hold settings that can be used to generate new documents from these template files. These settings include page margins, borders, headers, footers, and other page settings. Such templates are used in official documents such as company letterheads and standardized forms.
You can also convert OST into many other file formats. Please see the complete list below.