Use the -n option to check for installation errors. If -n does not display any errors, then omit the -n option when you install oracle-rdbms-server-12-1-preinstall.
If there are no errors, then log in as root, and install the group package:
# pkg install oracle-rdbms-server-12-1-preinstall
To view the packages installed by oracle-rdbms-server-12-1-preinstall:
TYPE FMRI
group x11/diagnostic/x11-info-clients
group x11/library/libxi
group x11/library/libxtst
group x11/session/xauth
require compress/unzip
require developer/assembler
require developer/build/make