Tropospheric ozone (O<sub>3</sub>) is a significant anthropogenic climate forcer with uncertain distribution in the Southern Hemisphere due to sparse observations. This study analyzes 28 years of in situ ozone, methane, carbon monoxide, and meteorological data at Tololo (30.17° S, 70.80° W, 2154 m a.s.l.), Chile, integrating reanalysis and atmospheric chemistry modeling. Here we identify a rising ozone trend of 2.1 ± 0.8 ppbv per decade since 2006, primarily driven by increasing background methane. We quantify contributions from biomass burning and stratosphere-to-troposphere transport, each adding approximately 5 ppbv per event during late winter and spring O<sub>3</sub> maximum. Stratospheric intrusions are linked to synoptic-scale troughs and cutoff lows, modulated by El Niño Southern Oscillation phases. These findings enhance understanding of ozone variability in the Southern Hemisphere free troposphere and underscore the importance of sustained observations at Tololo to monitor tropospheric ozone dynamics amid climate change.