In the Shanghai dialect dirty is pronounced something like "o co" with the o pronounced like the "o" in cot and the "c" pronounced like the usual c in pinyin (kinda like an 'ts' sound).
Which characters would be used for writing these two?
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
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.