In present work, the hydrodynamic studies and determination of oxygen mass transfer coefficients (KLa) were conducted in a multistage flexible fibre biofilm reactor developed for treating milk processing wastewater. In this regard, the residence time distributions in the MS-FFBR under various operating conditions as water flow rates, airflow rates, and hydraulic retention times were analysed using tracer experiments. The results revealed that the reactor’s hydraulic regime is similar to a continuous stirred tank reactor. Furthermore, the multistage flexible fibre biofilm reactor exhibited a lower oxygen mass transfer coefficient (KLa of 11.955 1/h at AFR/WFR of 47) than that reported for continuous stirred tank reactor in the literature at similar WFRs and AFRs (KLa of 15 1/h at AFR/WFR of 40). From the results, dissolved oxygen transfer was hindered to some extent owing to the presence of the fibre packing.