While relatively easy for a trained eye to tell apart with the "finer" lines of the Ki-43's fuselage – especially towards the tail – and more tapered wing planform, in the heat of battle, given the brief glimpses and distraction of combat, Allied aviators frequently made mistakes in enemy aircraft identification, reportedly having fought "Zeros" in areas where there were no Navy fighters.

Like the Zero, the radial-engined Ki-43 was light and easy to fly and became legendary for its combat performance in East Asia in the early years of the war.

It could outmaneuver any opponent, but did not initially have armor or self-sealing fuel tanks, and its armament was poor until its final version, which was produced as late as 1945.

The specification called for a top speed of 500 km/h (310 mph), a climb rate of 5,000 m (16,000 ft) in five minutes and a range of 800 km (500 mi).

These changes involved a major weight saving program, a slimmer fuselage with the tail surfaces moved further aft and a new canopy.

Crucially, the 11th prototype introduced the unique differential "butterfly" maneuvering Fowler flaps, which dramatically improved performance in tight turns.

The 13th prototype combined all these changes, and tests of this aircraft resulted in an instruction for Nakajima to place the Ki-43 into production, the Ki-27 jigs being transferred to the Mansyu factory at Harbin in Japanese occupied Manchukuo.

The first unit equipped with the Ki 43-I was the 59th FR at Hankow Airfield, during June–August 1941 and began operational sorties over Hengyang on 29 October 1941.

[7] Like the Zero, the Ki-43 initially enjoyed air superiority in the skies of Malaya, Dutch East Indies, Burma and New Guinea.

As the war progressed, however, the fighter suffered from the same weaknesses as the slower, fixed-gear Ki-27 "Nate" predecessor to the Oscar, and the more advanced naval A6M Zero; light armor and less-than-effective self-sealing fuel tanks, which caused high casualties in combat.

As newer Allied aircraft were introduced, such as the Republic P-47 Thunderbolt, Lockheed P-38 Lightning, North American P-51 Mustang, Vought F4U Corsair, Grumman F6F Hellcat, Yakovlev Yak-9, Yakovlev Yak-3U and late-model Supermarine Spitfire/Seafire, the Japanese were forced into a defensive war and most aircraft were flown by inexperienced pilots.

From October to December 1944, 17 Ki-43s were shot down in air combat; their pilots claimed seven C-47s, five Consolidated B-24 Liberators, two Spitfires, two Bristol Beaufighters, two de Havilland Mosquitoes, two F4U Corsairs, two Boeing B-29 Superfortresses, one F6F Hellcat, one P-38, and one North American B-25 Mitchell.

After the war, some captured examples served in limited numbers in the French Air Force in Indochina against Viet Minh rebels.

[26] Ki-43s abandoned in the Netherlands East Indies were taken over by the newly declared Indonesian government and put into service during the fight against Dutch forces.