this is known issue when running oim_blkld.sh command to load users or roles
to fix this find ojdbc5.jar at your machine(this jar file be a lot of places)
at linux use below command to find this jar file
$locate ojdbc5.jar
past it under /lib
cp ojdbc5.jar <IAM_HOME>/server/db/oim/oracle/Utilities/oimbulkload/lib
rerun the utility. it should work now.