Prediction of bubble size distributions (BSD) is challenging but necessary to develop a more advanced bubble population-based model for oxygen mass transfer with increased data quality while reducing the required experimental effort. In this paper, we experimentally investigated bubble sizes in a pilot-scale setup using a submersible in-situ flow-microscope. This technique enables bubble size measurements in dense bubbly flows and at airflow rates of up to 40 sL/min above the diffuser, where former measurement methods were limited to a range below 8 slpm with a comparable diffuser configuration. The data obtained were used to study coalescence and breakup behavior and to predict bubble size distributions using population balance modeling (PBM). We also investigated the prediction of the volumetric oxygen mass transfer coefficient based on measurement of bubble size at only one height in combination with PBM to provide reliable estimates of the apparent mass transfer rate with less experimental effort. A mass transfer rate estimate was obtained with deviations from the experimentally determined mass transfer rate of <10 %.