In this paper, we analyzed and considered the similarity and difference between Japanese noun sentences and verb sentences, focusing on “Prefix O + Conjunctive Noun + Auxiliary Verb” (hereinafter “O-Noun Sentence”). Despite the fact that the syntactic form of the same type is often used in the real life as a concise honorific expression, it has surprisingly not received much attention in existing studies related to honorifics. However, among the few studies, Shinya (2003, 2014) picked up “O-da” corresponding to the same form as a typical example of Japanese noun orientation and noun-centered syntax, and compared it with other honorific expressions. Through analysis, she approached its morphological and syntactic features. However, Shinya’s analysis was aimed at the combination of the prefix ‘o’ and the conjugation ‘da’ that constitutes the same form, so analysis and consideration on the verb conjunctive form, which is another component, is not enough. Therefore, in this paper, we newly define the verb conjunctive form appearing in “O-Noun sentences” as “conjunctive nouns”, and then examine the grammatical behavior in the same form, including the validity of combining with prefixes.