Air has 21 % oxygen by volume. In a pond at 303 K, the Henry's law constant of oxygen is 46.82 kbar…

Step 1. Partial pressure. $p = 0.21 \times 1.00 = 0.21\ \text{bar}$.

Step 2. Mole fraction. We want $x$, so $x = \dfrac{p}{K_H} = \dfrac{0.21\ \text{bar}}{46\,820\ \text{bar}} = 4.49 \times 10^{-6}$.

Step 3. Moles. $4.49 \times 10^{-6} \times 55.6\ \text{mol} = 2.49 \times 10^{-4}\ \text{mol} = 0.249\ \text{mmol}$.

Step 4. Mass. $2.49 \times 10^{-4}\ \text{mol} \times 32\ \text{g/mol} = 7.97 \times 10^{-3}\ \text{g} = 7.97\ \text{mg}$.