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The important idea here is shelf life and how oxidation affects essential oils. Citrus oils are especially prone to oxidation because they are rich in limonene, a compound that oxidizes quickly when exposed to air, light, or heat. When limonene oxidizes, it forms hydroperoxides and other products that change the scent and can irritate the skin, so these oils lose their quality faster. That’s why citrus oils typically remain best for about nine months to a year if they’re stored properly.

In contrast, many wood and resin oils have more stable chemical structures and oxidize more slowly, giving them a longer practical shelf life. Floral oils can be delicate, but the hallmark shorter shelf life tied to rapid oxidation is most strongly associated with citrus oils, making them the best answer for this question. To extend citrus oil freshness, keep them in dark glass bottles in a cool, dark place and limit exposure to heat and light.