An Orbital Defense Generator, or ODG, is a large groundside power plant used to generate energy for a planet's Orbital Defense Platforms. Prior to the Fall of Reach, Reach had seven ODGs to power its twenty ODPs. As demonstrated during the battle, ODGs were vulnerable to ground assault, and the failure of UNSC ground forces to protect the power plants and the resulting nullification of the planet's ODPs was a major factor in the battle's outcome and resulted in the loss of Reach. There were also ODGs on Earth to power the Orbital Defense Platforms around the planet.