In sunlight, the Moon's surface reaches about 390 K…

Step 1. Hydrogen. $v_{rms} = \sqrt{\dfrac{3 \times 8.314\ \text{J/(mol K)} \times 390\ \text{K}}{0.002\ \text{kg/mol}}} = 2205$ m/s.

Step 2. Oxygen. Sixteen times heavier, so four times slower: $\dfrac{2205\ \text{m/s}}{4} = 551$ m/s.

Step 3. The fractions. For hydrogen, $v_{mp} = 1801$ m/s and $x = \dfrac{2380\ \text{m/s}}{1801\ \text{m/s}} = 1.32$. The fraction is $0.322$: about a third of the hydrogen molecules can escape at any moment. For oxygen, $x = 5.29$ and the fraction is about $4 \times 10^{-12}$.