Diffusion is the dominant physical mechanism for the transfer of oxygen into fine-grained aquatic sediments. This diffusive uptake occurs at the sediment-water interface, but also at internal interfaces, such as along ventilated burrows or O2 releasing plant roots. Here, we present a systematic model analysis of the oxygen transfer at such biological interfaces. We list the equations for the O2 distribution, the flux, the oxygen penetration distance (OPD), the oxygenated sediment volume, and the irrigational oxygen uptake (IOU) as a function of biological parameters, such as burrow/root radius and burrow/root density. We also provide a set of computational "recipes" indicating how these model expressions can be used in the analysis of experimental data. As an example application, we show that the observed OPD reduction around ventilated burrows is largely due to the geometric effect of interface curvature. Because of this curvature effect, root and burrow surfaces cannot be treated as simple extensions of the sediment-water interface. As a general rule of a thumb, the burrow or root radius must be larger than OPD at the sediment-water interface to safely neglect geometrical corrections. Burrow and root systems in coastal environments typically do not meet this criterion.