The cells membrane is selectively permeable, meaning only certain substances can easily pass through it. Only small, non polar molecules can easily pass through the small spaces between the hydrophobic heads and phospholipids. Water would have trouble due to it being a polar molecule. Proteins and carbs are quite large, making it difficult for them to pass through. Salt, which ionizes water, would also have difficulty passing through as it is a charged particle. Oxygen, on the other hand, is a non polar molecule that is small, so it can easily pass through the phospholipid bilayer.
