Aerated treatment wetlands are an increasingly recognized nature-based technology for wastewater treatment that relies heavily on mechanical aeration. Although aeration-mediated oxygen transfer into the wastewater can be impeded by wastewater pollutants, little is known about the link between the volumetric oxygen mass transfer coefficient k_La and the organic carbon concentration of the wastewater in aerated wetlands. In this study, oxygen transfer experiments were carried out in a lab-scale gravel column using clean water and wastewater from a pilot-scale horizontal flow (HF) aerated wetland treating domestic sewage. The α-factor, which describes the ratio of the volumetric oxygen mass transfer coefficient k_La in wastewater to clean water, was reduced by increasing soluble chemical oxygen demand (COD_s). The derived regression equation α=1.066–1.372E-3 mg COD_s L^–1 was incorporated into a numerical process model to simulate the impact of the reduced oxygen transfer on a hypothetical HF aerated wetland. The simulations revealed that α and treatment efficacy for nitrogen were substantially reduced by COD_s at low aeration (k_La of 1 h⁻¹) and high influent wastewater strength (COD_s of 300 mg L⁻¹). At the same k_La and influent COD_s concentration, longitudinal gradients of α and concentrations for dissolved oxygen (DO), NH₄-N and NO_x-N in the simulated wetland were shifted up to 21% of wetland length downstream. These effects decreased with increasing k_La and were found to be negligible at k_La  > 3 h⁻¹, which corresponds to an air flow rate of approximately 400 L m⁻² h⁻¹. Following this, higher organic carbon concentrations can reduce oxygen transfer in HF aerated wetland systems, thus resulting in decreased treatment efficacy.