In short, Ozone = OpenMRS 3 + (“OpenMRS 3 plus“).
Ozone augments OpenMRS 3 by combining it with other health information systems, thereby bringing many more features to frontline health workers such as patient billing, pharmacy dispensing, a laboratory system, and many more.
OpenMRS 3 is the central EMR component of Ozone HIS.  See what is Ozone.