text: when do plants give off oxygen?   in general, plants give off oxygen when there is sunlight for photosynthesis. they respire all the time (use up oxygen), but during the day this is outweighed by the oxygen they produce.  some plants use cam photosynthesis which changes the answer. they are able to close their stomata (pores) day and night during harsh conditions. this means that though they are photosynthesising, no oxygen is being released. they're just using it up internally. they can't do this forever, but it does mean they can have periods of no oxygen release. some desert and tree-dwelling plants use this system.