I'd like to open this thread to document discussions and writings on the subject of "Oshikiuchi" as incorporated into Daito ryu.
Many people accept Stanley Pranin's explanation in his book CWDRM, page 22:
This is definitely a possibility, especially if you believe that Saigo did not study martial arts, as his diaries seem to imply."Tokimune and others have written that Sokaku learned secret techniques called oshikiuchi and that it was these arts that form the essence of Daito ryu. The characters used for oshikiuchi, "O (an honorific) + shiki (ceremony) + uchi (inside)," [3 kanji provided] represent a rather curious combination, and do not convey any obvious meaning. They were probably recorded based on the oral testimony of Sokaku who was himself illiterate. One theory is that the correct Chinese characters are actually [4 kanji provided] "O (an honorific) + shikii (threshold) + uchi (inside)." According to this view, what was actually referred to as oshikiuchi were not martial techniques at all, but rather court etiquette or manners that trusted subjects of the inner circle who were allowed "inside the threshold" were expected to observe. If this is indeed the case, what Chikanori Hoshina [aka: Saigo Tanomo] taught Sokaku during the latter's visits had to do with matters of samurai etiquette."
However, as Mr. Pranin notes, the majority of direct students of Sokaku concur that oshikiuchi was - or did include - self defense techniques, and were taught to Sokaku by Saigo Tanomo.
As far as kanji goes, if oshikiuchi was a term orally transmitted within Daito ryu, then the kanji could easily be confused. Sokaku was said to have spoken a little unclearly at times, I believe because of his front teeth having been knocked out.
The kanji that has been recorded for oshikiuchi (in the first example on page 22) is interesting. The first kanji "O" can be considered an honorific, or, it can be pronounced "Go" which refers to a palace. This "O/Go" kanji is the same one used in "GOten", which means a "palace or court". Gotenjutsu, a term used sometimes in reference to aspects of Daito ryu, may in fact be an alternate name for Oshikiuchi. Also, Oshikiuchi and Goshikiuchi sound similar, and it is possible that this was the intended pronunciation.
The first set of kanji on page 22 for "shiki" is very close to that of "bu" as in "BUdo"), which if used, would more clearly state that this was a combative art to be applied inside the palace (gobu-uchi??). This similarity in kanji could be a coincidence, or it could be a copiest error. It depends on whether the term was transmitted orally or written by hand in mokuroku.
"Uchi" (inside) is the only kanji not in contention.
Oshikiuchi has been classified in some texts as "Otome waza/jutsu/bujutsu", which refers to an official art associated with a certain clan and typically "secret". Otome in the context of Daito ryu would be used as an alternate term as opposed to something completely different.
Any thoughts?