This chapter provided an introduction to ONOS. After covering the ONOS architecture in brief, we showed how ONOS can manage Open vSwitch. With OVS being the most-used virtual switch in OpenStack deployment, this step was important to understand the relationship between ONOS and OpenStack. We touched upon growing an application for the SDN framework on the service provider network. We also described how to install and configure an ONOS system and outlined the landscape where ONOS fits in the context of OpenStack and XOS. Finally, we summarized the role of XOS as a service orchestration layer unifying OpenStack management.