This one is a regular mistake. Oils are BOTH sealants and moisturizers. Or, rather, certain oils are. The best way to keep your hair moisturized is to know which oil is which and how to use it accordingly.
Quick primer: A sealant keeps the moisture inside the cuticle by creating an outer layer. But if you use oil on your nonmoisturized hair, you will end up with dry ends very quickly. That's because the oil has no hydration to lock into the strand! Try using coconut, jojoba, or castor oils to add moisture, while olive, avocado, and argan oils are great sealants. On wet hair, apply a leave-in conditioner and follow up with the oil of your choice.