Shigeta submitted his answer as I was writing this!
Sanseveria is one of a wide group of plants (mainly succulents) that adopt a photosynthetic strategy referred to as crassulacean acid metabolism (CAM).
Recall the basics of photosynthesis. The light-dependent reactions use energy from captured photons to generate ATP and NADPH, with the generation of O2. In the light-independent reactions (the Calvin cycle) this ATP and NADPH is used to fix CO2 to form three carbon sugars, which can then enter standard metabolic pathways to produce other metabolites.
In order to obtain CO2 for the fixation reactions plants have pores in their leaves called stomata to promote gas exchange (via diffusion). This presents a challenge for plants that grow in arid conditions: if they open their stomata during the day they lose water as water vapour. Many such plants use CAM to solve this problem. Basically what these plants do is to only open their stomata at night, capturing CO2 in the form of organic acids (oxaloacetate, malate, maleate) which are stored in vacuoles. Then, during the day, these acids can be metabolised to release the CO2 again for use in the photosynthetic reactions.
Notice that in this scheme the O2 will still be generated during the day. So when does it leave? Does it have to wait until the stomata open at night, or is it able to diffuse out through cells (recall that O2 readily diffuses through biological membranes)? I have been unable to find a definitive answer to this question so far (this is old literature), but my guess is that it is lost throughout the day.