Urban O₃ trends across Europe and the USA were explored between 2000–2021. Most warm season trends in high O₃ in Europe were shown to be decreasing, though low values in both warm and cold seasons were increasing. Trends in the USA were similar, but showed less increasing trends low O₃ in the warm season. Sites with similar trends were grouped, more different were seen in the USA, and identified ones where trends in high O₃ values had started to increase later in the period.