Then there must be some code somewhere that is loading the Oracle driver class. If you add "-verbose:class" to the JVM options in the run script, the JVM will print out the classes loaded and the JAR file each one came from. With this setting you should be able to see the driver being loaded at some point; unfortunately that will not help you identify what is loading the driver.
Just had another thought - look in the jmx console at what data sources are registered. Of course, this won't list cases where code is loading the driver.