Error 500: java.lang.NoSuchMethodError: oracle/jdbc/OracleConnection.createOracleArray(Ljava/lang/String;Ljava/lang/Object;)Ljava/sql/Array; (loaded from file:/sysap/oracle/instantclient_11_2/ojdbc6.jar by com.ibm.ws.bootstrap.ExtClassLoader@96620801) called from class com.model.dao.TypeKeysDAO (loaded from file:/opt/WebSphere/8_5/AppServerBase1/profiles/AppServerBase1/installedApps/System_Cell/MyEnterprise.ear/MyEnterpriseWeb.war/WEB-INF/classes/ by com.ibm.ws.classloader.CompoundClassLoader@3edbaa21
The old copy of the JDBC jar appears to be associated with a JDBC provider defined in your configuration. The error message indicates that the class was loaded by WebSphere's ExtClassLoader, which is the loader that contains JDBC driver class paths. You'll need to remove the old JDBC provider, update its class path, or make it an "isolated" resource provider (which gives it a separate class loader that must be explicitly associated with an application) in order to get it out of the view of your application.
If there's some technical reason that you need ojdbc6.jar in the resource provider class path but need to reference ojdbc8.jar just within your application (not through a server-configured datasource), then you'll need to do some class loader configuration wizardry to make it work. The most reliable solution would be to create a shared library containing the new jar, set it to use an isolated class loader, and associate it with your EAR or WAR.