We experimentally demonstrated an efficient optical parametric oscillator (OPO) with high parametric conversion from 1.0645 to $3.8\mathrm{\mu m}$ . An aperiodically poled magnesium oxide doped lithium niobate wafer was designed and fabricated as the nonlinear crystal of the OPO. A linearly polarized acousto-optic Q-switched $\mathrm{Nd}\text{:}{\mathrm{YVO}}_4$ laser was used as the pump source. High pump-to-idler conversion efficiency of 18.5% was achieved with a slope efficiency of up to 21.5%. When compared with a periodically poled channel fabricated on the same wafer, under the condition of output coupler optimized for the periodically poled lithium niobate based OPO, an improvement of slope efficiency by 28.3% from 15.2% to 19.5% and total efficiency by 12.5% from 13.6% to 15.3% under the highest pump power of $11\mathrm{W}$ was realized for the pump-to-idler conversion.