Shake flasks are a widely used and incredibly useful bioreactor for bioprocess development and scale-up of mammalian and microbial bioprocesses. There are several parameters to consider when optimizing a bioprocess in a shake flask, including the conditions of the shaker platform (e.g. shaking frequency and orbital diameter) and the conditions of the shake flask (e.g. flask size and filling volume). The maximum oxygen transfer capacity (OTRmax) is a measure of the maximum rate at which oxygen transfers between the gas and the liquid phase in the vessel. Since most cells require oxygen to grow, this is generally viewed as a key predictor for the performance of a shaken bioprocess. Here we review how the OTRmax of a shake flask will vary as a function of both shaker conditions and flask conditions.