*** Guide-to-Links ***
NO is a connector given to words such as "um" and "oh" which have
no grammatical usages. It is important to list such words in the
dictionary, so that the parser does not treat them as "unknown
words" and try to use them as nouns, verbs, etc. By giving them
NO+ (and never using NO-), we ensure that they are always
null-linked, and thus excluded from the sentence.

Grammar Documentation Page.