Results from an 11-year simulation of the present-day atmosphere with a coupled tropospheric chemistry-climate model have been used to investigate the extratropical transport from the stratosphere to the troposphere, the transport and photochemical destruction of ozone from stratospheric origin in the troposphere, and the resulting contribution to tropospheric OH. The simulated stratosphere--troposphere exchange (STE) is based on the model calculated wind fields, and its seasonality and spatial distribution resemble results derived from other methods. The lifetime of stratospheric ozone in the troposphere is largely determined by transport between the extratropical tropopause where it enters the troposphere and the subtropical lower troposphere where its photochemical destruction rate maximizes. Comparison of the simulated ozone and O3s budgets suggests that ozone from stratospheric origin contributes about 15% to the average oxidation capacity in the Northern Hemisphere (NH), with regional contributions up to 40%. STE has a reverse effect on OH in the extratropical upper troposphere related to the influence of water vapor concentrations. The oxidation capacity of the troposphere may therefore be sensitive to atmospheric dynamic effects of climate change.