Both surface and satellite observations have shown a decrease in NO emissions in East Asian countries in recent years. In order to reflect the recent NOx emission reduction and to investigate its impact on surface O concentrations in Asian megacities, we adjusted two bottom-up regional emission inventories of which base years are 2006 (E2006) and 2010 (E2010), respectively. We applied direct and relative emission adjustments to both E2006 and E2010 to constrain NOx emissions using OMI NO vertical column densities. Except for the relative emission adjustment with E2006, modeling results with adjusted emissions exhibit that NOx emissions over East Asian megacities (Beijing, Shanghai, Seoul, and Tokyo) in the bottom-up inventories are generally overestimated. When the direct emission adjustment is applied to E2006, model biases in the Seoul Metropolitan Area (SMA), South Korea are reduced from 24 ppb to 2 ppb for NOx (=NO+NO) and from -9 ppb to 0 ppb for O. In addition, NO model biases in Beijing and Shanghai in China are reduced from 8 ppb to 18 ppb-0 ppb and 1 ppb, respectively. Daily maximum 8-h average O model biases over the same places are decreased by 8 ppb and 14 ppb. Further analyses suggest that the reduction in domestic South Korean NO emissions plays a significant role in increasing O concentrations in SMA. We conclude that the current strong drive to reduce NO emissions as part of the strategy to lower particulate matter concentrations in South Korea can account for increased O concentrations in recent years and suggest that more aggressive NO emissions will be necessary soon.