A method of fabricating an oscillating heat pipe includes building the oscillating heat pipe with a layer-by-layer additive manufacturing process such that the oscillating heat pipe includes a body of solid material, an array of channels, an evaporator portion, and a condenser portion. The array of channels are disposed in the body and define a continuous loop through which a fluid flows. The array of channels is formed by cavities in the body as the body is formed with layer-by-layer additive manufacturing. An inner surface of a channel includes a flow directing feature that is configured to promote a first direction of flow and that is configured to provide resistance against a second direction of flow that is opposite the first direction of flow.