Hi @olulani,
can you run the plugin directly from the commandline via MK_CONFDIR=/etc/check_mk /usr/lib/check_mk_agent/plugins/mk_oracle -t and see if the instances are dedected and login is possible (be warned, the output contains sensible data)?
Can you also please run sqlplus -V | grep ^SQL | cut -d" " -f3 | cut -d"." -f-2 as oracle user with environment?