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.
Fifteen years after Operation Costner, the OMDF was again thrust into war when the Yuktobanian Air Force and Yuktobanian Navy launched a raid on Port St. Hewlett, an Osean naval base and home of the 3rd Osean Naval Fleet, both by air and sea. The Wardog Squadron was sent to defend the fleet and its flagship, the Kestrel at any cost. Later on however, on September 30, 2010, a rendezvous in the Eglin Straits where the 3rd Osean Naval Fleet was supposed to regroup for a counterattack ended in disaster when ballistic missiles from the super-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 front-line base in Yuktobania. After this, the OMDF was likely responsible for safely moving Osean forces from Osea to Yuktobania.
On or around December 7, 2010, the Kestrel, as well as a number of other ships, probably from the 3rd Osean Naval Fleet, left the OMDF and formed the Kestrel Fleet. The Kestrel Fleet believed that the Belkan Gray Men were the true enemy, not Yuktobania. The Kestrel carried a squadron of four black fighters, Ghosts of Razgriz, who were launched on various missions against the Gray Men.
On December 29, 2010, during Operation Glory Horn, an Osean fleet led by the OFS Barbet, another OMDF aircraft carrier and the Kestrel's sister ship, was detected during an engagement between the Admiral Tsanev fleet and the Kestrel Fleet. Since the Kestrel Fleet had been working with three defecting Yuktobanian naval ships and under command of President Vincent Harling, the Osean fleet went rogue and attacked the Kestrel Fleet. However, the Barbet and her escorts were completely decimated by the Razgriz. The following day, on December 30, 2010, the Kestrel was attacked and sunk by an enemy submarine, however, the Razgriz and most of its crew were able to escape, and the enemy submarine was destroyed.
By the end of the Circum-Pacific War, the OMDF was depleted, having lost much of its carrier force.
Around ten years after the Circum-Pacific War, the OMDF, having rebuilt their strength after the war, 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 destroyer 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.
- Aircraft carriers
- Aegis cruisers
- Chivalry †
- 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)
- Thrush (Aegis)
- Lark (Aegis)
- Wild Goose (Aegis)
- Landing ships
- Iolite †
- Finch †
- File:3rd Osean Naval Fleet Emblem.png.
- This Means War, Ace Combat 7: Skies Unknown
- PS2/エースコンバット５ | BACKGROUND | 設定資料. Retrieved February 28, 2015.
- Commander, Third Fleet. Retrieved on 5 October 2017.
- Juggernaut, Ace Combat Zero: The Belkan War.
- Cutscene 02: "This Means War", Ace Combat 7: Skies Unknown.
- Cutscene 03: "444th Air Base", Ace Combat 7: Skies Unknown.
- Mission 05: "444", Ace Combat 7: Skies Unknown.
- Mission 03: "Two-pronged Strategy", Ace Combat 7: Skies Unknown.
- 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 19: "Lighthouse", Ace Combat 7: Skies Unknown.
- Mission 20: "Dark Blue", Ace Combat 7: Skies Unknown.