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 ODS extension stand for OpenDocument Spreadsheet Document format that are editable by user. Data is stored inside ODF file into rows and columns. It is XML-based format and is one of the several subtypes in the Open Document Formats (ODF) family. The format is specified as part of the ODF 1.2 specifications published and maintained by OASIS.
You can also convert OST into many other file formats. Please see the complete list below.