When attempting to use Splunk DB connect, please note that the Splunk documentation was out of date at the time of this writing.
The only two things you need for the Splunk DB connect to work is ojdbc6.jar copied into $SPLUNK_HOME/etc/apps/dbx/bin/lib and the latest JRE downloaded to a directory and the path set for the application.
The error messages that the Splunk Application provided were cryptic and either verifying/testing the connection when adding a new DB connection or when trying to pull data from the Application after setting up the DB Connect profile.
The simple solution was to use the latest Java JRE. The version that worked, even though the Splunk documentation referenced earlier version would work was JRE 7u55. In my case, jre-7u55-linux-x64.tar.gz on a Redhat Linux 64bit OS.
These did not solve any of the Splunk DB Connect error messages are are misleading:
Other things that are important. If you install the oracle client, it will typically install the following JAR files.
... and the version 6 files with a similar name.
Here is what I would suggest to try:
1) Reinstall db connect app from apps.splunk.com; restart splunk
2) Download ojdbc6.jar and place it into $SPLUNK_HOME/etc/apps/dbx
3) Restart splunk, and retest connecting to Oracle database.
4) Download the latest JRE (version 7).