水と油はまざらない。

water (esp. cool, fresh water, e.g. drinking water); fluid (esp. in an animal tissue); liquid; flood; floodwaters; water offered to wrestlers just prior to a bout; break granted to wrestlers engaged in a prolonged bout

if; when; and; with; particle used for quoting (with speech, thoughts, etc.); quoting particle; promoted pawn; indicates question (sentence end)

oil

topic marker particle; indicates contrast with another option (stated or unstated); adds emphasis

to excel; to surpass; to exceed; to have an edge; to be superior; to outrival; to outweigh; to preponderate