Oxygen is transported across the plasma membrane through diffusion. Diffusion is the random movement of particles down their concentration gradient from an area of higher concentration to an area of lower concentration. It is the simplest method of transport through a plasma membrane, and it does not require energy input.
The plasma membrane creates a barrier between a cell and its surroundings. It is made up primarily of a phospholipid bilayer with the hydrophillic phospho-headgroups facing out along the surface of both sides, while the hydrophobic hydrocarbon tails form the interior. The lipid bilayer prevents large macromolecules or polar molecules from traveling into the cell. Smaller, nonpolar molecules such as hormones, oxygen, carbon dioxide and ethanol are not repelled by the hydrophobic portion of the bilayer, so they diffuse freely into or out of the cell dependent on their concentration gradient.
Transport through the plasma membrane can be either passive or active. Examples of passive transport include diffusion, facilitated diffusion and osmosis. Osmosis is specifically the diffusion of water. Facilitated diffusion is the use of a passageway to allow larger molecules to follow their concentration gradient. This type of diffusion requires the assistance of a carrier molecule. Active transport requires the use of energy to allow molecules to travel against their concentration gradient.