Oxygen molecules are small, and all cells need it for respiration to release energy from food molecules. Because there's a higher concentration of oxygen molecules outside the cell compared to the cell's cytoplasm, you'll see that oxygen diffuses into the cell along this concentration gradient.

Why does oxygen diffuse into cells rather than out of them?

Internal Respiration

Oxygen diffuses into tissues because its concentration is higher in the blood than inside the tissue (where oxygen is always being used up during cellular respiration). Carbon dioxide, which is always being produced during cellular respiration, diffuses out for the opposite reason.

Does oxygen easily diffuse into cells?

Only the smallest molecules like water, carbon dioxide, and oxygen can freely diffuse across cell membranes. Larger molecules or charged molecules often require an input of energy to be transported into the cell.