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.
Enhanced metafile format (EMF) stores graphical images device-independently. Metafiles of EMF comprises of variable-length records in chronological order that can render the stored image after parsing on any output device. These variable-length records can be definitions of enclosed objects, commands for drawing, and graphics properties critical to render the image accurately.
You can also convert OST into many other file formats. Please see the complete list below.