Onan oshi (横難横死)
Onan (横難) oshi (横死) means to die in an unforeseen disaster or to die an unnatural death.
Japanese "o" (横, side) is phonetically similar to Chinese "o" (枉, to bend) and has the same meaning as bending into a sidetrack (横道に逸れる), including the following meanings:
Something not to be straight or out of the right way. Unrighteousness, not to be the main point.
Therefore, each meaning is as follows:
onan (横難) means sudden or unforeseen, unreasonable, and unexpected disaster. and oshi (横死) means sudden or unforeseen, unreasonable, and unexpected death. That is, it means to die an unnatural death such as by murder, disaster, accidents, and so on, without fulfilling one's own destiny. A violent death, and so on.
Also, "oshi" (枉死, unnatural death) is used in Tenmonshige (astronomical reports in China) in "Gokanjo" (History of the Later Han Dynasty), and so on, while 'yokosama no shi' (横様の死, unnatural death) used as Wago (words of Japanese origin) corresponds to oshi (横死) when it is read in the 'kun' or Japanese reading.
Also as a synonym of this word, there is obyo (横病, unexpected illness), which has a similar meaning to sudden illness or suffering an unexpected disease.
Nichiren is an example of a person who frequently used the word, onan oshi.
Nichiren, advocating Shikakakugen (four dictums blaming other sects), mentioned that if one depends on heresy, not on Hoke-kyo Sutra (the Lotus Sutra) or Nam Myoho Renge-kyo Sutra (Devotion to the Law of Hokekyo), one would encounter onan (横難) or oshi (横死) as retribution.
For example, Nichiren says, 'There is nothing superior to sansho (three norms to be used for discrimination between right and wrong in religion), and thinking of Zenmui and Ichigyo who suffered onan (横難) oshi (横死), and the ways Kukai and Ennin died, I wonder how true practitioners of shobo (the true teachings of Buddha) could be like them' (in his "Kyogyosho gosho," Kyogyosho-writings or letters of superior) and so on, and denies founders of the Singon Sect and so forth.
Also he says as follows:
In Nehan-gyo Sutra (The Sutra of The Great Nirvana), it says, "Suffering shio (death in disaster) unfairly (横しまに), one receives keiho (light punishment of this world for one's bad karma of the past world) such as kashaku (repentance), meniku (condemnation and insult), benjo (whip and baton), kiga (hunger) and inku (quarrel and trouble), and does not fall into hell," and so on...'
Besides, he mentions that the essential point of the sutras is doing us wrong, although we practiced shobo in the past, but now if we believe and practice the shobo again, the kudoku (merit) gained by practicing shobo in this world is so strong that we will deserve only a small amount of trouble without inviting larger troubles in the future, even if we should fall into great hell in the future due to sins we committed to hurt others in the past' (in "Kyodaisho" (Nichiren's teachings given to Ikegami brothers in 1275). Stated above, suffering onan (横難) oshi (横死) sometimes leads to light punishment for bad karma of one's past hobo (slander of the Law) (this is called Tenju kyoju, practicing shobo allows one to receive one's heavy bad karma of the past world lightly, or sometimes even lets it disappear) and so on.
However, unnaturalness means not to depend on goin (right or wrong conduct which can be a cause of the happiness or trouble in the future) of the past world in Buddhist terms. Therefore, unnatural death in principle refers to an unexpected death suddenly encountered in this world without depending on goin of the past world. Also in various Buddhism dictionaries and so on, oshi (横死) is defined as ending one's life without depending on one's goka (good or bad results caused by one's own karma) of the past world.
Kushuoshi (unnatural death of nine kinds or nine causes)
In Buddhism it is explained, with difference in each kyoron (sutra), that there are nine kinds or causes in oshi (横死). This is referred as kushuoshi (unnatural death of nine kinds or nine causes).
Theory of "Yakushi-kyo Sutra"
Even when one becomes ill, one can not receive medical treatment. One is executed by the national law for committing vice. When one is devoted to araku (great easiness) and does not behave oneself, one is deprived of energy by devils and specters while one is unguarded. One is burned. One falls into water and drowns. One is eaten by evil beasts in mountains and forests. One lose one's life by falling from a cliff. One suffers poisoning. One dies suffering from hunger and thirst.
Theory of "Kyuoshi-kyo Sutra"
One eats although one should not do. One eats too much. One eats what one is not used to. One eats before food is digested. One holds it in. One follows and experiences worldly customs without practicing the precepts of Buddhism. One approaches bad company. One enters into mountain villages when one is not allowed. One passes over plague or mad dogs without avoiding them.