If you installed OIF 11g on OEL 6.2 64bit and passed all the interesting gotchas but got stumped by this error in the WLS admin logs, and Enterprise Manager refuses to start correctly after what appeared to be a full successful install and configuration:

 ...  <User defined listener oracle.sysman.eml.app.ContextInitializer failed: java.lang.NoClassDefFoundError: HTTPClient/ProtocolNotSuppException.
java.lang.NoClassDefFoundError: HTTPClient/ProtocolNotSuppException
    at oracle.sysman.eml.app.ContextInitializer.contextInitialized(ContextInitializer.java:1035)
...

Caused By: java.lang.ClassNotFoundException: HTTPClient.ProtocolNotSuppException
    at weblogic.utils.classloaders.GenericClassLoader.findLocalClass(GenericClassLoader.java:297)
    at weblogic.utils.classloaders.GenericClassLoader.findClass(GenericClassLoader.java:270)
...

<Error> <Deployer> <BEA-149231> <Unable to set the activation state to true for the application 'em'.
weblogic.application.ModuleException: 
    at weblogic.servlet.internal.WebAppModule.startContexts(WebAppModule.java:1520)
...

The problem is the installer fails to properly place a required jar (http_client.jar) in the appropriate location for your WLS domain.   Assuming you have Oracle DB installed on the same server, just copy the jar to the lib folder in your domain, e.g. if your domain is IDMDomain and middleware install location is /u01/Middleware then

cp /u01/app/oracle/product/11.2.0/db_1/oui/jlib/http_client.jar \

  /u01/Middleware/user_projects/domains/IDMDomain/lib

and restart your admin WLS.  Enterprise Manager will start to work.  Hopefully this will save others some time while on the bleeding edge...