How to install Oracle's sqlplus on a Mac?
Download Basic Package and SQL*Plus Package zip files from Oracle
Add a local repository to brew from where the Oracle packages will be installed: brew tap InstantClientTap/instantclient
Copy both Oracle's zip files to /usr/local/Homebrew/Library/Taps/instantclienttap/homebrew-instantclient
Install the basic package: brew install instantclient-basic
Install the SQL*Plus package: brew install instantclient-sqlplus
Test sqlplus by connecting to an existing database: sqlplus user/password@server:port/sid

My intention was to improve Joost van Wollingen's article published here.