The Army, being a critical component of the Osean military, has seen combat in several conflicts. In March and April 1995, with the outbreak of the Belkan War, they halted and repulsed a Belkan invasion of Osea's recently acquired Great Lakes territories, before deploying airborne troops to help liberate the allied nation of Ustio in May. Following the Allied resolution to inspect and strip Belka of nuclear weapons, the Osean Army invaded South Belka and, after initial opposition, began occupying demilitarized towns as the Belkan Army retreated north. The Osean advance was only halted with the June 6th Nuclear Attacks, which sealed the Waldriech passes to North Belka an ended major ground operations in the war.
Following the nuclear attacks and the subsequent ending of hostilities in its wake. Osea restructured its standing military forces into the Osean Defense Forces, or ODF. The Osean Army officially became known as the Osean Ground Defense Force (OGDF).
The Osean Army established a beachhead on the Bastok Peninsula over a month after the start of the Circum-Pacific War. In the month that followed, the Osean Army rapidly advanced through central Yuktobania, capturing strategic positions such as the Jilachi Desert. By December, the Osean Army had successfully captured Cruik Fortress and was beginning preparations to attack Cinigrad.
However, the offensive subsequently stalled in the face of intense resistance, at least partially due to insufficient air support, and the war entered a stalemate. The stalemate would continue until December 30, when President Vincent Harling and Prime Minister Seryozha Viktrovich Nikanor held a peace talk, calling for their respective armies to fight against the true enemy responsible for the war, the Grey Men. Elements of the Osean Army subsequently moved to Sudentor to support the offensive, and the Grey Men were soon defeated.
A large detachment of the OGDF operating under the callsign(s) Menhir and commanded by Major Deanna McOnie was responsible for providing area security at Stonehenge during Operation Dragon Breath. Engineers from the OGDF were also responsible for restoring and firing one of the railguns. With the assistance of the LRSSG, the OGDF successfully prevented Stonehenge from being overrun by Erusean forces, allowing the weapon to successfully destroy Arsenal Bird Liberty.
Osean Army units formed the bulk of the forces that laid siege to Farbanti during the pivotal battle. Despite facing stiff Erusean resistance, with aid from the Long Range Strategic Strike Group and an off-shore Osean fleet, they were able to take Farbanti. However, their victory was short-lived, as Osean and Erusean anti-satellite attacks caused a communications black-out that affected the War.
OGDF forces were among the stranded combatants that were part of the fighting at Anchorhead. It would only be thanks to the efforts of AWACS Long Caster that they were able to navigate out of the combat zone. Also, during that battle, an Osean Special Forces unit led by Captain Karl attempted to extract Édouard Labarthe, an Erusean general who wished to defect. Even though Karl's unit successfully escaped the city, they were mistakenly shot down by OADF jets who were unaware of the mission and were misled into believing Karl was an Erusean impostor.
- 1st Army Battalion
- 2nd Army Battalion
- 1st Tank Battalion: (Bishop; Eagle & Hawk)
- 2nd Tank Battalion: (Black Knight; Bulldog; St. Bernard; Dachsund & Dalmatian)
- 3rd Tank Battalion: (Lynx; Burmese)
- Tank Battalion(s): (Coblynau & Beagle)
- Howitzer Brigade (Dragonfly)
- Ferdelance Armored Brigade (Dragon; King Cobra; Tarantula; Puff Adder; Mamba; Huntsman; Taipan; Python & Abel)
- 101st Airborne Division
- 112th Infantry Battalion
- 1st Airborne Battalion
- Tucker Team
- Wand Squadron
- Helm Squadron
- M1A1 Abrams
- Heavy Expanded Mobility Tactical Truck
- M113 APC
- M998 Humvee
- Generator Truck
- Stryker APC
- M16A1 Assault Rifle
- Mk. 18 Mod 1
- M9 Pistol
- SIG P250
- M47 Dragon
- The Osean Ground Defense Force's uniform and equipment is for the most part the same used by the United States Army circa 2004, much as how the Osean Air Defense Force and Osean Maritime Defense Force have similar equipment to the United States Air Force and Navy, respectively.
- In addition, the 101st Osean Airborne Division is a reference to the American 101st Airborne Division.
- The Osean Ground Defense Force appears to draft civilians for specialist duties. This can be seen in Operation Dragon Breath, where it is mentioned that the survey team responsible for tracking the Arsenal Bird's movements are civilians, and likely students.
- Mission 12: "Stonehenge Defensive", Ace Combat 7: Skies Unknown.
- Mission 16: "Last Hope", Ace Combat 7: Skies Unknown.
- Mission 02: "Annex", Ace Combat Zero: The Belkan War.
- Lit Fuse, Ace Combat 5: The Unsung War.
- Desert Arrow, Ace Combat 5: The Unsung War.
- Desert Lightning, Ace Combat 5: The Unsung War.
- Fortress, Ace Combat 5: The Unsung War.
- Cutscene 17, Ace Combat 5: The Unsung War.
- Ace Combat 5: The Unsung War.
- Cutscene 26, Ace Combat 5: The Unsung War.
- Aces, Ace Combat 5: The Unsung War.