Oracle driver not found error can happen because of many reasons .Below are few of the scenario.

1. Please check if the external library is placed in the correct directory.it should be in “/opt/streamsets-datacollector/streamsets-libs-extras/streamsets-datacollector-jdbc-lib/lib/”.In case this path does not exist, create it.

2. We recommend to install external drivers from the same SDC UI  by going to your pipeline> Select the stage > External libraries > Install External Libraries and then restart your SDC.

3. Please check if there are multiple jdbc jars with different versions are in use for different libraries as this could add some issue on how Linux distributes the jars. Linux gives back the drivers in a non-deterministic order, and it can lead to this situation in very rare cases.