I had found out that, on some on my compute running OL7 on OCI-C, the cron job did not source the proper .bashrc environment file as I would have expected, where all ORACLE variables were set.
Workaround is to add the 1 or 2 last of the following lines into the cronjob:
CRON_TZ=Europe/London SHELL=/bin/bash BASH_ENV=/home/oracle/.bashrc