Particulate matter is usually regarded as the dominant pollutant in Tehran megacity in Iran. However, the number of ozone exceedance days significantly increased in recent years. This study analyzes simultaneous measurements of O₃ and NO_x (NO+NO₂) concentrations to improve our understanding of ozone evolution during the summers of 2017 to 2019. The k-means clustering technique was used to select five representative air quality monitoring sites in Tehran to capture O₃ and NO_x concentrations' variability. The findings show that all of the investigated sites failed to meet the ozone non-attainment criterion. The ozone weekend effect is seen in the study of weekday/weekend differences in 2017 and 2018, but not in 2019, which can be due to the shift in the ozone production regime. The summer mean variation analysis can also be used to deduce this regime change. In 2017, the O₃ and NO₂ summer mean variations suggest a holdback in the NO₂ upward trend and a reversal in the O₃ downward trend that had been in place since 2012. Air mass back trajectory clustering reveals that east and north-east air mass clusters have the most significant impact on Tehran's O₃ pollution and the highest regional contribution to O_X. The study of O_X against NO_x shows that the regional contribution to O_X increased from 2017 to 2018 and then decreased in 2019; however, the local contribution is the opposite. The diurnal analysis of the regional and local contributions to O_X indicated that O_X in Tehran might be primarily affected by pollutants from a short distance. The findings reveal critical changes in the behavior of O₃ in recent years, indicating that decision-makers in Tehran should reconsider air pollution control measures.