KOSHIGAYA Gozan's 1775 dialect dictionary Butsurui shōko ("Names for things, by type") on how to speak when spoken to, wherever you are:

In the Kantō region, they say ai. In the Kinai region, they say hai. In Ōmi, they say nei. Around Nagato, they say attsu. In Satsuma, they say ō. In Hizen, they say nai. In Tosa, they say ei (they also say etsu, and servants and the like say ō and yatsu). In Echigo, they say yai, and in Echizen, they say yatsu. In Mutsu, they say nai.

The replies of the various provinces listed here are basically the same, so despite the slight variations they must be related. Among them, ō is used in all areas to reply to inferiors, but it can be used when speaking to superiors in some parts of Kyūshū. This ō is commonly written 應 [modern 応], but since ō is a native Japanese word, it should really be written 唯. With this the traditional authorities, too, agree (先哲も沙汰し侍る).

In other words, writing ō with 應 would be using the Chinese pronunciation of the character to evoke the sound, even though the meaning of the character is different. This is an abuse; really, you should use 唯, which has the meaning (among others) of a reply to a summons, but sounds completely different (in Chinese, that is). It is no doubt indicative of my moral failings that I find this logic elegant and persuasive.

(Listen: you can't just be using characters for sound alone. The last time people did that, Japanese ended up with two separate syllabaries, each with hundreds of redundant characters that it took a thousand years to boil away. There have to be rules. We live in a society. With this the traditional authorities, too, agree.)

Ahem. After a couple of usage examples from classical sources (including "漢書二　唯唯　注：恭（ツツシンデ）應（コタフル）ノ詞ト有", natch), Koshigaya closes with this hokku by Kyorai:

"Ō, ō," to/ ihedo tataku ya/ yuki no kado

"Alright, already" / comes the reply, but the knocking continues-- / gate in the snow