Round Metallic Object. RMOs are collectible coins used in drinking challenges by members of the armed forces and some civilian law enforcement and emergency services groups.
In initiating a challenge, one throws down their RMO. If all others present an RMO as well, the initiator buys drinks. If someone in the group does not present their RMO, they buy the drinks.
Capt. Shitdick threw his RMO on the one day I didn't have mine on me, so I ended up buying the whole squadron drinks.