I noticed that some of my oils would lose their efficacy faster than the listed shelf life. Oils eventually go bad but extending their properties is easy.
-Keep them in the dark bottles they come in. Sunlight denatures oil.
-Store them in cool places away from heat. High temperatures are the enemy of oil.
-Keep the bottles tightly closed after use. Air promotes rancidity.
-If you transfer your oils to transparent bottles, keep them in a dark place like your closet or dresser.