Nematic order, a self-organized state with rotational symmetry broken, has been observed in both copperoxide and iron-pnictide high temperature superconductors. However, its origin is still a mystery in the iron pnictides although it is considered as a key to understand the mechanism of superconductivity. Here, we report a systemic nuclear magnetic resonance (NMR) study on NaFe(1-x)C(o)xAs (0 <= x <= 0.042) that an orbital order, accompanied by an instant spin nematicity, occurs at at a temperature T* far above structural transition temperature Ts in the tetragonal phase. We show that the observed NMR spectra splitting and its evolution is due to an incommensurate orbital order that sets in below T* and becomes commensurate below T-s. We show that the electric field gradient asymmetry parameter is a good measure for the orbital order parameter which undergoes a Landau-like 2nd-order phase transition. We further show that the spin nematicity is well accounted for by the observed orbital order.
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