I'm astounded to see a series of the answers and comments to this question. Most of them are misguiding and confusing the O.P. They lack even a beginner's knowledge of Chinese language about how to read it and what it means.
None of “応、鳴、唯, 越々, and 応々” corresponds to the pronunciation equivalent to “おう” – "oh" or "ou." They are pronounced respectively as "ying," "ming," "wei," "yue-yue" and "ying-ying" in Chinese. And all suggested words are pointless and utterly wrong. If you have just an elementary knowledge of Chinese language, you wouldn't make such a primitive and laughable mistake.
They are absolutely different beasts from “さあ,” “まあ” and “おう" either phonetically or semantically. If you have any objections, please contend with me to make your case. I'm waiting for your rebuttal.
Both “応" and "鳴" are verbs each meaning "respond" and "sing, chirp, twitter." "唯” is an adjective meaning "only, sole" in both Japanese and Chinese, ”越々” is an adverbial phrase meaning "the more ... the more," all irrelevant to “さあ,” “まあ,” and “おう" which are interjections.
The connection of "応々" in 去来's haiku with Chinese word is comically farfetched. There's no Chinese word spelt or vocalized like 応々, and if it should happen to be by chance, it must be read "ying-ying," not "ō, ō" as an answerer suggested.
I think @sqrtbottle is right. And only he was right.
If Japanese, “さあ” and “まあ” is a loan word from Chinese word or character that sounds “oo” as Samuel Martin says, it may be “哦” which is pronounced as "o" or "oh."
A Chinese language dictionary at hand, “多功用常用字典” - published by 新華出版社 in Beijing - defines “哦” as a "感嘆詞.表示懐疑或吃惊. 哦! 是这么回事. – [Exclamation] indicates a suspicion or surprise. Example. Oh, my God! What is this all about! "
However, I’m not sure whether “さあ” and “まあ” are really a loanword from Chinese language.