Chamber with semi permeable membrane

Consider two cases in a chamber in which gases are filled on both sides of a partition which acts a semi permeable membrane :
1.Hydrogen is filled on one side of the partition and it can move through the semi permeable membrane.Hence it will flow till the pressures on both sides are equal.
2.Next,I have hydrogen on one side and oxygen on another.Oxygen cannot penetrate the membrane,but hydrogen is allowed.Still it is observed that hydrogen flows till it’s partial pressure becomes same on both sides.
Why does it happen like that in the second case?Why do the oxygen molecules not contribute in stopping the hydrogen molecules from diffusing into the semi permeable membrane?