Once you have your essential oils in the bottle, it’s time to add the carrier oil. Unlike essential oils bottles, carrier oil bottles don’t usually have an orifice reducer, so you’ll probably need a pipette or small funnel. Add carrier oil to the top of the main part of the bottle, but not into the neck where the ball goes. Then, insert the roller ball and its holder into the neck of the bottle. Once you’ve done that, it’s ready for you to put the cap on the bottle.