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.
A file with .emz extension is a compressed container of Enhanced Metafile (.emf file). It follows .GZIP compression algorithm and takes less storage space due to compression. The small file size makes it more easy to transfer EMZ files over the network.
You can also convert OST into many other file formats. Please see the complete list below.