The changes of different O(3) metrics such as O(3_avg) (daily mean), O(X) (NO(2) + O(3)), O(3_8h) (daily maximum 8-h moving average), O(3_max) (daily maximum 1-h average), and exceedances of O(3_8h) and O(3_max) at 16 air quality monitoring stations over northern Taiwan from 1994 to 2007 are evaluated. The O(3_avg) at the background site displays an increasing trend at 0.54 +/-0.21 ppb (1.28 +/-0.49%) per year. The O(X) at urban sites shows a significant increase, indicating that even with the effect of reduced NO titration increasing photochemical production is evident. The exceedances of O(3_8h) (>80 ppb) are about 14.9% higher than O(3_max) (>120 ppb) with more exceedances at coastal or background sites, where the anthropogenic O(3) precursor emissions are lower. In addition to increasing background O(3) and reductions in NO titration, increasing O(3) production efficiency also plays a key role in the increase of moderate to high O(3) concentrations. The findings help explain the different trends in exceedances and levels of O(3_8h) and O(3_max) and are likewise important in the formation of O(3) control strategies.