Make sure that the cap is on securely and the bottle stored up-right in a cool dark place. The oils should be stored out of sight and the touch of children. Remember that children, especially small ones, are very inquisitive. Never store essential oils in plastic bottles. Good Essential oils should keep for several years if properly stored, though the oils of orange, lemon and lime will not keep as long.
Patchouli is at the other extreme and actually gets better as it ages.
It is strongly recommended that you use a dropper so that you can measure the actual number of drops easily. Use a different dropper for each oil to avoid cross contamination. The Droppers supplied in bottles should be in different sizes according to the viscosity of the different oils. Good internal droppers have a grove on one side. Great after a hard day.