ROS isn't really an operating system but a framework to combine robotics-related code into one running system via a publisher/subscriber model. You'd typically run it on ubuntu as the actual OS

you actually can get an (90%) autonomous robot off the shelf, if you just want a differential-drive base with a lidar or kinect sensor. Most of the work is done for you in the ROS modules, it's just a matter of knowing which ones to use, installing them and getting them to communicate over the correct channels.