Physiologically, both chronic and cycling low oxygen level, or hypoxia, can alter tumour cell behaviour and reduce effectiveness of anticancer treatment. In order to create transiently hypoxic environments around tumour cells during drug screening, short diffusion distances between the control gases and the culture media are required in order to reproduce the timescales of a few cycles per hour that have been observed in animal studies. We use a gas-permeable three-layer microfluidic device to achieve spatial and temporal oxygen control with equilibration time less than 10 minutes. We integrate thin optical oxygen sensing films composed of polystyrene with an embedded luminescent oxygen-sensitive dye in order to monitor oxygen levels within the system. We show that the microfluidic oxygen control afforded by this design is sufficient to achieve oxygen concentrations below the sensor limit of detection of 0.08% in the fluidic cell culture environment. This platform allows us to generate complex, time-varying oxygen profiles. The sensors are biocompatible and engineered tumour spheroids cultured in this platform show good viability and cell proliferation over the course of several days with in situ oxygen monitoring.