Japan and Thailand were the countries that used the Ki-43.
The first 40 Ki-43-I-Hei were in service by December 1941.
The Nakajima Ki-43 was used in Burma, China, the Philippine Islands, Thailand, and the Japanese home islands.
It was equal to the Allied fighters early in the war but was soon outclassed by the newer Allied designs.
The Ki-43 was first used in the Malayan, Netherlands East Indies, and Burma campaigns.
The Royal Thai Air Force received twelve Ki-43s and these were used against American planes in southern China.
In September 1943 the Allies had captured several Oscars and were able to construct a complete one that they tested and were able to figure out how to counter it's exceptional maneuverability at low speed.
The Ki-43s were often used in kamikaze missions by 1945.
Post World War II
Many Ki-43s survived to be used by the Indonesian forces against the Dutch. French pilots used some Ki-43s in Indo-China.