We perform an in-plane optical spectroscopy measurement on iron-based superconductor Li0.8Fe0.2ODFeSe single crystal. At room temperature, the low frequency optical conductivity shows an incoherent characteristic; the Drude component is absent. With temperature decreasing, the Drude component develops and narrows rapidly. A well-defined plasma edge is observed in reflectance spectrum at temperature below 100 K, indicating a dramatically reduced scattering rate. The spectral weight contributed from free carriers is even smaller than that of FeSe single crystal. A number of phonon modes are visible in the measured spectra. We also observe clear spectral change below 160 cm-1 at 10 K, associated with the formation of superconducting energy gap in the superconducting state. The energy scale of the superconducting gap is comparable to the value measured by angle-resolved photoemission spectroscopy technique. Like FeSe and other iron pnictides, a clear temperature-induced spectral weight transfer at high energy is observed for Li0.8Fe0.2ODFeSe, indicating the presence of strong correlation effect.