Aside from some difference in packages/prereqs, it's much the same as installing the agent on Linux. You can do the agent deploy from the Console, or various other advanced installation types. http://docs.oracle.com/cd/E24628_01/install.121/e22624/install_agent.htm#EMBSC181
The documentation does a good job of displaying the steps to deploy the agent to a target. The biggest problem that I seem to be having is identifying an agent for 12c and hp-ux. The OMS is installed on a linux server but we have databases running on hp-ux platforms that we would like to monitor/manage from the console.
This is covered in the Prereqs under Software Availability. You have to download any non-Linux agents since your OMS is on Linux.
In short, be sure that your Software Library is configured, figure out if you are running in Online or Offline Mode for Self Update, and then follow the docs on acquiring additional agent images: