 The Landsat-8 operational land imager, OLI, was calibrated before launch using an integrating sphere traceable to national institute of standards and technology, NIST, standards of spectral irradiance. It was also calibrated on orbit using diffusers characterized prior to launch and compared to other instruments and ground measurements. The calibration coefficients were provided with the data products to convert from radiance to reflectance units. This article was authored by Brian Markham, Julia Barsi, Gierke Farron, and others.