Essential oils are fun to explore. When I first ventured into the realm of aromatherapy I wanted to buy dozens of oils. But they can be expensive so you should protect your investment. You will want proper storage of your oils so they maintain their therapeutic properties.
Oxygen is one of the biggest threats to essential oils. If an essential oil oxidizes it will deteriorate and lose its beneficial properties. Oxidization can also make an oil more prone to causing skin irritation. The confusing aspect is that some oils, like citrus, can oxidize in only 6 months while some oils, like patchouli, are less prone to oxidation and actually get better with age. But, eventually, all oils will oxidize so they all can benefit from proper storage.
Oxidization is caused by oxygen reacting with the molecules of the essential oil, so reducing contact with oxygen is key. Don’t leave the cap off of the bottle for more than the few seconds it takes to measure out the liquid. And when you recap the bottle make sure the cap is screwed on tight. Also, when you have used up half of the essential oil in a bottle, be sure to decant the remainder into a smaller bottle. If you leave the oil in a large bottle then the oil has to share space with the oxygen that will fill the top half of the bottle.
Keeping essential oils in dark glass, such amber or cobalt blue bottles, will protect them from sunlight which can cause deterioration. Glass is also preferred over soft plastic because essential oils can eat through plastic causing the plastic to leach into the oil. This also means to avoid rubber dropper caps. Many years ago I purchased an oil in a glass bottle with a rubber bulb cap. The essential oil diffused through the cap and when I checked the bottle in 6 months the essential oil had completely evaporated!
The last step to prolonging the life of your essential oils is to store them in a cold, dark place. When kept from heat and light the oil will last longer. This means that your bathroom counter is NOT a good place to store essential oils. I have stored my oils in a box in a cool closet, in the refrigerator, and, more recently, in my cool Colorado basement. It may seem less convenient but it can mean the difference between an off-smelling oil that can cause irritation and an oil that smells and works beautifully for many extra months.