Aeration accounts for the largest energy demand in conventional activated sludge wastewater treatment. Emerging aeration control strategies for energy conservation have significantly reduced operational bulk liquid dissolved oxygen (DO) from above 2 mg/L to at or below 0.5 mg/L. As we move toward low DO treatment processes, there is a need to understand how low DO impacts the kinetics of micropollutant biotransformation. The objective of this study was to characterize the impact of DO concentration on pharmaceutical biotransformation rates via two approaches: (1) Determine oxygen half saturation constants that describe the community-wide impact of DO on biotransformation rates. (2) Evaluate shifts in the microbial community 16S rRNA pool due to DO concentration. Batch experiments were performed at several DO concentrations using biomass from a full-scale wastewater treatment plant. Results reveal that substantial reductions in bulk liquid DO concentrations to 0.5 mg-O2/L are possible without compromising pharmaceutical biotransformation rates. Sequencing of cDNA generated from community rRNA revealed that diverse, low abundance community members may play important roles in pharmaceutical transformation. The results of this work advance our ability to predict and model the impact of DO on pharmaceutical biotransformations during wastewater treatment and identify taxonomic groups associated with those biotransformations.