Recent developments in the area of biological detection by optical sensing of molecular oxygen (O2) are reviewed, with particular emphasis on the quenched-phosphorescence O2 sensing technique. Following a brief introduction to the main principles, materials and formats of sensor technology, the main groups of applications targeted to biological detection using an O2 transducer are described. These groups include: enzymatic assays; analysis of respiration of mammalian and microbial cells, small organisms and plants; food and microbial safety; monitoring of oxygenation in cell cultures, 3D models of live tissue, bioreactors and fluidic chips; ex vivo and in vivo O2 measurements; trace O2 analysis. For these systems, which enable a range of new bioanalytical tasks with different samples and models in a minimally invasive, contact-less manner, with high sensitivity, flexibility and imaging capabilities in 2D and 3D, relevant practical examples are presented and their merits and limitations discussed. An outlook of future scientific and technological developments in the field is also provided.