As answered concisely by StarCub, 齷齪 龌龊 is the Hanzi representation for Shanghainese
o co. Yet IMHO to call this word "the Mandarin equivalent" of
o co is a bit inappropriate, since
- from my understanding you are just asking for a Hanzi representation for a dialectal word, yet not its "equivalent" (or synonym, IMHO). A common mistake is to neglect the fact that any Chinese dialect can be a standalone system, completely represented in Hanzi yet pronounced in a sound system different from Putonghua but still well organized in a way derived from ancient Chinese.
- though "dirty, messy" appears as one explanation for 龌龊 in modern Putonghua dictionaries, it's far less frequently used (from my daily experience) than the word's extended meanings "mean, despicable" or "narrow".
To add up, I'd like to mention 腌臢 腌臜
a1 za1 in at least 贛 Gan dialect
nga41 tsa is also a high-frequent daily word expressing "dirty". And the phonetic similarity among 腌臢
nga tsa in 赣, 齷齪
o co in 吴, 污糟
wu tsou in 粤 and 肮脏
ang tsang in Putonghua is really a interesting topic for further discussion.