Solar-heating siphon-assisted oil recovery is promising as an eco-friendly strategy for oil spill mitigation due to its spontaneous, continuous, and renewable operation. However, the slow oil transport rate has constrained its progression. Here, we report a novel “legs-to-head” directional U-shape freeze casting method to fabricate siphon-assisted oil skimmers with well-aligned channels in the “legs” and interconnected pathways in the “head” to enhance the oil recovery rate. Real-time temperature monitoring during freezing clarified the influence of the connecting angle between the “legs” on the channel structure within the “head”. Resulting from a balance between the transport barrier in the “head” and the flow resistance in the “legs” of the oil skimmers, optimal performance was achieved at a 60° connection angle between the “legs”, yielding a high oil recovery rate of 620.2 L m⁻² h⁻¹ under 1 sun irradiation. Furthermore, the outdoor test demonstrated a peak rate of 938.2 L m⁻² h⁻¹ at noon and a remarkable daily collection of 14 001 L m⁻², achieved without external power or human intervention. Notably, the oil flow flux is calculated to be 207 218 L m⁻² h⁻¹ bar⁻¹, outpacing conventional pump-assisted devices. This work provides pioneering designs towards efficient oil spill remediation and other fluid transport applications.