。
Oil is immiscible with water.
Literal
Oil [topic-は] water [with-と] not-mix.
混和する (こんわする) is the technical/scientific term for miscibility — more precise than the everyday 混ざる or 混じる. The kango compound (混 'mix' + 和 'harmonize') gives the sentence a chemistry-textbook feel; the と marks the partner element rather than a quotative.