I think you mean carbon dioxide instead of oxygen. During photosynthesis, plant leaves absorb carbon dioxide from the air and transfer it to the chloroplasts for processing. The mesophyll of a plant is the tissue layer in the middle of the leaf between the epidermis layers. It consists of a palisade layer which contains the majority of the chloroplasts. There is a small space between the cells which allows for CO2 movement and absorption. Underneath this is the spongy layer which has even larger air spaces between the cells. The stomata, or pores of the leaf, open into substomatal air chambers in the spongy layer. The stomata are what allow CO2 to enter into the mesophyll tissue of the leaf.