Gas molecules move with a typical speed vrms = √(3kT/m), with k = 1.38 × 10⁻²³ J/K.

Step 1. Hydrogen. $3kT = 3 \times 1.38 \times 10^{-23}\ \text{J/K} \times 700\ \text{K} = 2.90 \times 10^{-20}\ \text{J}$, so $\sqrt{\dfrac{3kT}{m}} = \sqrt{\dfrac{2.90 \times 10^{-20}\ \text{J}}{3.3 \times 10^{-27}\ \text{kg}}} = 2960\ \text{m/s}$.

Step 2. Oxygen. $\sqrt{\dfrac{2.90 \times 10^{-20}\ \text{J}}{5.3 \times 10^{-26}\ \text{kg}}} = 739\ \text{m/s}$.

Step 3. Compare. Hydrogen's typical speed is about $70\%$ of the escape speed. Oxygen's is about a sixth of it, still enough: the fastest molecules in the spread leak away over millions of years.