It’s a post-installation configuration that hasn’t been done. The /etc/sudoers file has the following section in it:

In the default (unconfigured) configuration, sudo asks for the root password. This allows use of an ordinary user account for administration of a freshly installed system. When configuring sudo, delete the two following lines:

Defaults targetpw   # ask for the password of the target user i.e. root
ALL ALL=(ALL) ALL   # WARNING! Only use this together with 'Defaults targetpw'!

The administrator needs to edit /etc/sudoers, using the visudo command, and comment out the final two lines above, or remove them altogether. Once done, individual user accounts can be added as follows:

oracle ALL=(ALL) ALL
norman ALL=(ALL) ALL

Alternatively, groups can be given access as follows:

%dba ALL=(ALL) ALL
%oinstall ALL=(ALL) ALL

Substituting the appropriate levels of commands and privileges of course!