The Olympian Army is made up of all the gods and demigods who fought for the Olympians in the Second Olympian War. Their aim was to defeat the Titan Kronos and the other Titans and gods that sided with him.
- Charles Beckendorf: Sacrificed himself to destroy the Andromeda, temporarily halting Kronos' invasion.
- Lee Fletcher: Died in the Battle of the Labyrinth.
- Michael Yew: Died in the Battle of Manhattan.
- Bianca di Angelo: She died during a battle with a Talos prototype. Her body was never found.
- Zoë Nightshade: She was turned into a constellation by Artemis after being mortally injured by her father.
- Leneus: He had split his stomach, fatally injuring himself before dying and turning into a Laurel sapling.
- Castor: Died an untimely death in the Battle of Labyrinth.
- Daedalus: Willingly chose to pass on into the Underworld.
- Silena Beauregard: She died trying to fend off a Drakon while dressed as Clarisse La Rue, so that the children of Ares would join the fight. Silena was hit with poison in her face by the creature and eventually died.
The Olympian Army was formed by cyclopes, demigods, satyrs, gods (in this case - Olympian), nymphs, and many other species to counter the Titan Army led by Kronos. The Greek Camp Half-Blood war leader is Percy Jackson and the second-in-command next to him is Annabeth Chase, his girlfriend. The satyrs are led by Grover Underwood, their protector and friend and the centaurs (the Party Ponies) are following Chiron, Camp Half-Blood's activities director and trainer.
The army was also intended to block Kronos' attempt to enter the Empire State Building, which was the only entrance and exit to and from Mount Olympus, in Manhattan so that he could not destroy the Olympian gods' thrones on Olympus. This blockage forced Kronos to reconsider into fighting his way towards the Empire State Building entrance. The battle that the demigods, satyrs, and the wood nymphs was known as the Battle of Manhattan.
Kronos had the entire borough of Manhattan and downtown New York City in a tight grip by forcing around it a big siege hole that forced back mortals traveling into NYC by vehicle and by plane under a time spell. Hecate also used her magic to cloak NYC and prevent mortals from entering or leaving. Morpheus, the god of dreams, joined the side of the Titan Army and put every mortals that were already in NYC or living in New York City into a deep slumber.
After meeting Percy on the steps of the Empire State Building with pretty much most of the campers, Annabeth and the other campers join Percy into the special Olympian elevator that took them to the supposed 600th floor of the Empire State Building which was Olympus. There they visit for a while but there is no point staying because there was a war looming up sooner than later.
Once back in Manhattan, the first defense that Percy came up with was to divide each campers that came along with Annabeth into groups under the patronages of their Olympian parents (see the groups above under Demigod campers of Camp Half-Blood list) to defend the main bridges that lead into Manhattan from the main part of New York State to block Kronos'attempt to get in pass the bridges. Eventually the army wins the battle when Luke Castellan sacrifices himself to scatter Kronos.