The hydroxyl radical (OH) plays a key role in tropospheric chemistry as the main atmospheric oxidant. However, studies suggest that atmospheric models typically overestimate the abundance of tropospheric OH, which adversely impacts our ability to reliably estimate the atmospheric lifetime of carbon monoxide (CO), methane (CH4), and other environmentally important trace gases. In this talk I will review the multi-decade effort to better quantify the oxidative capacity of the atmosphere, starting with early space-based measurements of CO. I will discuss recent work to integrate the current suite of space-based observations of atmospheric composition to better quantify tropospheric OH.