Often the question pops up "do essential oils expire or are they good forever?" And the short of it is no they don't really expire but no they're not good or safe to use topically forever either. As I have discovered with all things aromatherapy nothing is a hard and fast yes or no. Is your brain scrambled yet? ;)
Essential oils once opened begin to oxidize. The oxygen molecules in the air get into the bottles and change the structure of the essential oils. Citrus oils are especially susceptible to this process and that's why you see it recommended to not use citrus essential oils after 6-12 months topically. They are still perfectly good in your diffusers, cleaning products, linen sprays and such but topically they could become irritating to the skin.
There are many other essential oils that get amazing with age, like fine spirits or wine, their aroma is beautiful and they are powerful additions to your creations. One such oil is Patchouli. There are plenty of bottles of Patchouli running around that are 3 or more decades old and they are lovely.
In returning to our question, do oils expire? Not really, they oxidize or if they are essential oils combined with other oil, for example coconut oil, grape-seed, petrochemicals etc yes they will go rancid because of the addition of the oil they are mixed into.
How to best store you oils? Cool, dry, dark locations. Cabinets away from sunlight or cold storage like a temp controlled fridge are best.
If I have a citrus oil like Lemon and it's been open longer than a year what do I do? Smell it. Does is smell off from your normal lemony smell? Use it is cleaning products, diffuser blends or the like instead of topical use. Simple as that.
Where do you store your essential oils? Do they last long enough you worry about oxidation or do you use them up quickly? Be sure to share below I love to know what works best for you!