The IEEE Software Defined Networks' (SDN) eLearning Module 3 "It's All About Control: ONOS SDN Controller" builds on the previous two modules, "An Introduction to SDN: Part 1" and "An Introduction to SDN: Part 2", which covered SDN and SDN controllers, in general. Module 3 takes a close look at one particular Open Source Controller-- the Open Network Operating System (ONOS). This session demonstrates how ONOS fits into the SDN universe; what and how it controls and what problems it solves. In addition, it covers what ONOS brings to the SDN revolution.