Oils, any oils, are occlusive by nature. Occlusives essentially act as a barrier, wrapping around skin or hair ideally keeping the good stuff in and the bad stuff out. This is why you top your water-based toners and lotions with an oil—you are sealing in the hydration and nutrients underneath. This is a good thing.
It becomes a problem, however, when said oil doesn’t have any moisture to lock in—and then may be actively keeping water out. This brings us back to usage: Hair oils should be layered over water (be it on damp hair or from a water-based spray or cream), so they have conditioning agents to seal in; that’s the first issue. The second is that using too much hair oil may be harder to wash off in the shower (especially if you opt for gentle, sulfate-free shampoos or co-washes). So then, because you’re not fully washing off the product, water from your shower is repelled, unable to soak in, leaving your hair dry, dehydrated, and brittle.
(An addendum to this: For those with very high-porosity hair, the above scenario may work in your favor, as your strands tend to absorb too much water—causing swelling and breakage. But for those with average to low-porosity hair, oil buildup will lead to brittleness.)
So why do I single out coconut oil here? Because coconut oil is the one most likely culprit. There are a few reasons for this. The first, “Coconut oil tends to solidify some,” says board-certified dermatologist Raechele Cochran Gathers, M.D. (You know how it’s often solid at room temperature? Yeah, sort of like that). As it dries into a solid, it can cause strands to feel stiffer and more coarse—leading to breakage.
But coconut oil also is high in lauric acid, which naturally attracts and binds with hair proteins. This will cause protein buildup, leading to less elasticity and flexibility of hair—or common symptoms of brittleness.