OBIEE is capable of connecting to and retrieving data from a variety of data sources.  The type of data sources that OBIEE can connect to are OLTP, OLAP, Data Warehouses (ROLAP), and Files.

• OLTP databases – these include the normalized-design databases including ERP, CRM and other LOB systems.
  – The relational databases supported are: Oracle databases, Microsoft SQL Server, IBM DB2, and Teradata Warehouse.
  – And the ERP/CRM sources supported are: Oracle E-Business Suite, Oracle Peoplesoft, Oracle Siebel CRM, Oracle JD Edwards, and SAP.  Note: any ERP/CRM system running on one the databases mentioned above can be supported, but those mentioned here are special ERP/CRM sources.
• OLAP databases – these include dimensional-databases including applications based on dimensional databses.
  – The OLAP datases supported are: Oracle Essbase, Oracle OLAP, Microsoft Analysis Services, and SAP Netweaver BI.
  – And the OLAP applications sources supported are: Oracle Hyperion Planning and Oracle Hyperion Financial Management.
• Dimensionally-modeled data warehouses – these are relational databases designed with a star-schema / dimensional model, on one of the 4 supported relational databases mentioned above.
• Files – Microsoft Excel, XML files, Flat files.

The data retrieval methods used to connect to these sources are:

• OLTP – SQL
• OLAP – MDX
• Data Warehouse – SQL
• Files – ODBC

OBIEE has the ability to connect to multiple of these data sources at the same time, and the data sources can be of the same or different types.  So, for example, an OBIEE Server can source data from an Oracle 11g Data Warehouse, and from an Oracle Essbase 11g OLAP cube at the same time, and join the data together for user consumption.
Similarly, file datasources can also be added to provide additional information, for example from an external source, and joined to data from the other sources mentioned above.  This “joining” of data is handled by the OBIEE BI Repository and BI Server.

To the end user accessing the data from a front-end tool (Analysis Editor / Answers), it seems like a single data source. That is one of the features that makes OBIEE such a great tool particularly for heterogeneous database environments.