Oka was 14 years old on August 6, 1945 and working in Hiroshima as a communications operator at an underground command centre of the Imperial Japanese Army.
After the bomb fell, she contacted another military unit in the city of Fukuyama east of Hiroshima, local media including public broadcaster NHK reported.
"Hiroshima is almost destroyed," she said. "We were hit by a new type of bomb."
Local media said she died of malignant lymphoma on May 19 in hospital in Hiroshima after spending years recounting her experiences of the bombing and its aftermath to visitors and students in the city.
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"We have lost another great eyewitness," Kajiya told AFP.
American B-29 bomber Enola Gay dropped its deadly payload, dubbed "Little Boy", over the city at 8:15 am local time.
It marked the first use of an atomic weapon and ultimately claimed the lives of some 140,000 people.
Some died immediately while others succumbed to injuries or radiation-related illnesses weeks, months and years later.
In May last year Barack Obama made a historic visit to Hiroshima, becoming the first serving US president to do so.