For most oils refrigeration is not necessary but can't hurt. With the blue oils you definitely should refrigerate if you want to slow the oxidation process and keep the blue color as long as possible. Chamazulene is what causes the blue color and when it oxidizes it will cause the oil to become green. When this happens it doesn't mean the oil is necessarily bad but just a sign that some of the chamazulene has reacted with air. My general rule of thumb is basically "if the oil smells good, it is good" LOL. You can also extend the lifetime of citrus oils by refrigeration as limonene and monoterpenes in general are susceptible to oxidation. When limonene oxide forms in a citrus oil you will know it, even a very small amount will cause a foul odor. The biggest enemy to the oils by far is oxygen. Keeping low amount of airspace in the bottle is crucial to preservation for long periods. If possible, you definitely want to change to a smaller container before the bottle is half empty. If you refrigerate a half empty bottle it may slow the oxidation process but it will not protect against it, the oxygen in the bottle will still react with the oil, just at a slower rate. Don't put the oils in the freezer because some of them have components that will crystalize out. Many times when components crystalize out at the bottom of an oil the person doesn't notice it then pours off the liquid on the top which won't be representative of what the total chemistry of the oil should be. To make matters worse, the oil left at the bottom will now be abnormally high in the crystalized component after some of the liquid is poured off, so even after you warm it back up and get it liquid, its chemistry will not be right. So its not really the freezing of the oil itself that is the biggest concern but the resulting changes in the relative percentages of key components that result if a person uses a partially frozen oil before its all brought back to room temperature and thoroughly re-mixed.