In Apollo 2015, Apollo essential uses its own SQLLite for catalog database; Apollo Adv/pro 2015 uses its SQLServer/Oracle/Postges as catalog database, and all database drivers come from JBOSS JAVA JDBC. There is no need for additional database client software for Apollo 2015.
In Apollo 2016, Apollo essential shares the same database with Apollo adv/pro, Apollo essential need additional database drivers if database server is not installed on the same server as Apollo. Because Apollo Essential is .NET application, Java JDBC driver comes with TOMCAT will not work with Apollo essential. So Apollo essential requires database .NET driver for all databases. Without proper .NET database driver, user will failed to configure catalog database for Apollo essential during configuraztion wizard.
For Oracle database, ERDAS APOLLO 2016 requires the installation of the Oracle Call Interface OCI client libraries and Oracle Data Provider for .NET (ODP.NET) on the ERDAS APOLLO server in order to use an Oracle catalog database, or to crawl vector features in an Oracle database. These components can be installed either through a custom Oracle Client installation or a 64-bit Oracle Data Access Components (ODAC) installation. ODAC is simplest and easiest way to get the required Oracle components on Apollo server.
NOTE: The latest ODAC for Oracle 12.02 will be backward compatiable, so it will work with Oracle 11g and 12g database.
The non-XCOPY version ODAC is reocmmended since everything will be setup automatically.
In case user choose XCOPY version, then here is the workflow:
Here is how to install 64bit ODAC 11gR2 XCOPY for Apollo 2016.
 Download ODAC112040Xcopy_64bit.zip, and Unzip @ Apollo 2016 server machine  read the “readme.htm”. Follow the instruction from readme to install ODAC.
 From command line with administrator previledge:
>install.bat all c:\oracle odac
 The install.bat will copy all DLLs to c:\oracle folder, and regist DLL in registry.
 After installation, you will find the following ODP.NET DLL that Apollo essential requires: C:\oracle\odp.net\bin\4\Oracle.DataAccess.dll
 Add both c:\oracle and c:\oracle\bin to system PATH environmental variable. So that Apollo essential can access ODP.NET dll from Path variable. Control Panel->System->Advanced System Settings->Environment Variables->System variables:
 Restart Apollo server, and now you can configure Apollo 2016 with Oracle as catalog