The Osean Maritime Defense Force (OMDF), colloquially known as the Osean Navy, is the naval branch of the Osean Defense Forces. Osean ships serving under the OMDF carry the prefix OFS (Osean Federation Ship).
The OMDF is one of the largest naval forces in the world and its fleets have an impressive battle record. The 3rd Osean Naval Fleet in particular participated in major conflicts at the turn of the 21st century.
The OMDF was formed sometime by the 16th century, when Admiral Stafford formed the 3rd Osean Naval Fleet. Stafford subsequently earned the nickname "Father of the Navy".
By July 1991, Osea had six Hubert-class aircraft carriers across its fleets. The OMDF issued a contract on July 29 to the Newport Sargo Shipyard for the construction of a seventh Hubert-class carrier, the OFS Kestrel.
Early in the Belkan War, Osea initiated Offensive Campaign No. 4101 to capture the Futuro Canal and secure a sea route for advancing further north into Ustio and Belka. The OMDF's 3rd Osean Naval Fleet took part in this operation and used the opportunity to give the OFS Kestrel a trial voyage. Ustio mercenary pilots, including Galm Team, assisted the fleet's advance by wiping out Belkan resistance. With the operation's success, the OMDF was able to perform naval transit from Osea into Belka for the rest of the war.
In the 2010s, the OMDF was deployed again when war broke out with Yuktobania. Yuktobanian Air Force and Yuktobanian Navy launched a raid on St. Hewlett, an Osean naval base and home of the 3rd Osean Naval Fleet. The Kestrel was able to escape the port thanks to the efforts of Wardog Squadron and VFA-206. On September 30, an ambush at the Eaglin Straits by the Yuktobanian submarine Scinfaxi sunk the carriers OFS Buzzard and OFS Vulture along with many escort vessels and carrier-based aircraft. Later in the war, the OMDF's LCACs shipped a tank battalion from the Osean Army to the Bastok Peninsula in Yuktobania, where they executed Operation Footprint, establishing a bridgehead in Yuktobania.
In December, the Kestrel, as well as a number of other ships, went rogue. The Kestrel uncovered that the Belkan Grey Men orchestrated the war. The Kestrel became the base of operations for Razgriz Squadron, who were launched on various missions against the Grey Men, including the rescue of President Vincent Harling.
On December 29, 2010, during Operation Glory Horn, an Osean fleet led by the carrier OFS Barbet, was detected during an engagement between the Admiral Tsanev fleet and the Kestrel Fleet. Since the Kestrel Fleet had allied itself with three defecting Yuktobanian naval ships, the Osean fleet went labelled them "traitors" and attacked. Admist the engagement, the Barbet, her strike group and the hostile Yuktobanian vessels were sunk by the Razgriz. The following day, the Kestrel was attacked and sunk by an enemy submarine, however, the Razgriz and most of its crew were able to launch, and the enemy submarine was destroyed.
After peace with Yuktobania was re-established, the Osean Federation began to rebuild its maritime fleet and began construction of new aircraft carriers. Eventually, a number of new carriers were commissioned, including a new OFS Vulture and the cutting-edge OFS Kestrel II.
Following their commissioning, the OMDF began development of a new class of carrier, intending to name each vessel after Osean war heroes. The first ship was initially planned to be christened the "Nicholas A. Andersen", before the name changed to the "Admiral Andersen".
Around ten years after the Circum-Pacific War, the OMDF found itself engaged with the Kingdom of Erusea's forces for control of the International Space Elevator. Once again, their power projection capabilities were limited after many of their aircraft carriers were rendered inoperable following Erusea's drone attacks on Osean naval ports.
Mothballed aircraft formerly used by the OMDF, including F-14D Super Tomcats and F/A-18F Super Hornets, were utilised by Spare Squadron to defend the 444th Air Base and in other engagements in Usea. These aircraft had their emblems painted over with 'sin lines', as was customary for other aircraft stationed at the base.
During Operation Dual Wielder, International Union Peacekeeping Force squadrons from Fort Grays Island linked up with the OFS Vulture for a joint mission to secure air superiority over the Chopinburg Rainforest. Unbeknownst to the IUN forces, however, Erusea managed to acquire the Arsenal Birds Liberty and Justice, deploying the former in an attempt to repel the aerial forces. During this time, the OFS Kestrel II was deployed to bomb Farbanti, but was ultimately sunk in the ensuing battle.
The OMDF later took part during the operations to seize the Alicorn, and was present in some capacity in two out of the three operations, Operation Sighthound and Operation Fisherman.. During Operation Sighthound, a detachment of ships led by the Puffin attempted to capture the Alicorn, only for all ships in the group to be crippled by the submarine's railguns, in spite of the LRSSG having guarded them while en-route to Artiglio Port.
On September 19, 2019, during Operation Giant's Step, a combined fleet of ships led by the OFS Stork engaged an Erusean naval formation tasked with defending Farbanti. With the combined efforts of Trigger and the Long Range Strategic Strike Group, the Osean fleet was able to overpower the fierce resistance of the remnants of the Erusean Navy. However, after the Osean Ground Defense Force began moving onto the enemy's general HQ, Sol Squadron entered the airspace in a final effort to drive the advancing Osean forces back; Mihaly A. Shilage began attacking the vessels preparing to forcefully dock, severely damaging the cruisers Hornbill & Shoebill, before moving on to engage the LRSSG.
On October 10, 2019, during Operation Reflux, a trio of Osean landing ships was retreating from Tyler Island. Amidst the chaos of the unreliable IFF due to the satellites being knocked out of commission, Jaeger noticed that one of the ships, Wagtail, was an Osean vessel and were forced to retreat and leave behind troops retreating at the beachhead. Despite the overwhelming odds, AWACS Long Caster identified the LRSSG as friendly and tasked them with providing immediate support to the forces under fire, and allowed stranded Osean forces to make their way to the landing ships.
During Operation Daredevil, a formation of Osean missile destroyers led by the cruiser Starling participated in the unified attack on the remaining Arsenal Bird, Justice, alongside the coalition of Osean and Erusean forces. Despite the superior firepower, Justice activated its Microwave Powered Dome, rendering the saturation attack useless. The Arsenal Bird then began using its forward-mounted laser to destroy the Starling, Drake, and Lark, forcing the rest of the remaining fleet to withdraw from the area. Following Justice's destruction, remaining forces were forced to retreat as two new advanced UAVs had entered the airspace, landing at the abandoned OFS Admiral Andersen aircraft carrier. The following day, they took off from the carrier to engage the UAVs.
- A-6E Intruder
- C-1 Trader
- CH-47 Chinook
- E-2 Hawkeye
- EA-6B Prowler
- EA-18G Growler
- F-14A Tomcat
- F-14B Bombcat
- F-14D Super Tomcat
- F/A-18C Hornet
- F/A-18E Super Hornet
- F/A-18F Super Hornet
- F-35C Lightning II
- Kawasaki P-1
- Rafale M
- Su-33 Flanker-D
The OFS prefix is excluded since all ships listed here implicitly carry it. Vessels that have been sunk or crippled are denoted by the † symbol.
- Other carriers
- Ticonderoga-class cruiser
- Hatakaze-class destroyer
- Chivalry †
- Cassard-class destroyer
- Arleigh Burke-class destroyer
- Babbler (Aegis) †
- Hamerkop (Aegis) †
- Kite (Aegis) †
- Nightjar (Aegis) †
- Parrotbill (Aegis) †
- Grebe (Aegis)
- Rooster (Aegis)
- Seagull (Aegis) †
- Crane (Aegis) 
- Egret (Aegis)
- Stork (Aegis)
- Quetzal (Aegis) †
- Sandpiper (Aegis) †
- Snipe (Aegis)
- Trogon (Aegis)
- Drake (Aegis)
- Lark (Aegis)
- Starling †
- San Antonio-class amphibious transport dock
- Oliver Hazard Perry-class frigate
- Iolite †
- Finch †
- Freedom-class littoral combat ship
- Most vessels in the OMDF are named after birds.
- The Admiral Andersen is the only known vessel in the OMDF that is named after a person.
- File:3rd Osean Naval Fleet Emblem.png.
- This Means War, Ace Combat 7: Skies Unknown
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- Mission 19: "Lighthouse", Ace Combat 7: Skies Unknown.
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- Cutscene 03: "444th Air Base", Ace Combat 7: Skies Unknown.
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- SP Mission 01: "Unexpected Visitor", Ace Combat 7: Skies Unknown.
- SP Mission 03: "Ten Million Relief Plan", Ace Combat 7: Skies Unknown.
- Mission 15: "Battle for Farbanti", Ace Combat 7: Skies Unknown.
- Mission 17: "Homeward", Ace Combat 7: Skies Unknown.
- Mission 20: "Dark Blue", Ace Combat 7: Skies Unknown.