Orbital Defense Cannons are massive railguns employed by the IMC Colonial Navy, for the purposes of orbital defence. They are situated on planets with strategic significance, and usually in overwhelming numbers.
As gargantuan, stationary weapons, ODCs are very high-value targets for an attacking force. As such, they are protected by garrisons of IMC Marines, though they are often known to be some of the "easiest postings in the service".
When preparing to fire, the gun must first perform a lengthy charge-up sequence. Once done, the weapon will fire a green-coloured bolt at the enemy vessel, causing extreme damage.
300 weapons of this type were stationed to defend the drydock used to repair IMS Sentinel, including Outposts 197, 207 and 210. The gun at 207 was captured by the Militia, and used to destroy the IMC carrier IMS Sentinel.
An ODC railgun can also be seen mounted on a satellite in the Satellite Strike callsign, indicating these weapons can also be used in space.
Multiple installations can be seen on King’s Canyon, as well as small versions being mounted on the supply ships. Railguns are also present on Olympus, on one occasion shooting down an inbound Goblin piloted by Pathfinder.