。
Oil and water just don't mix.
Literal
Oil [and-と] water [topic-は] not-mix [explanatory-んだ].
混ざる (まざる, 'to mix') is the everyday intransitive verb — much more casual than the technical 混和する. んだ adds an explanatory tone: 'the reason is, they simply don't mix.' Pairing 油 ('oil') with 水 ('water') and と as a conjunctive between nouns lists them as the joint topic.