Heat KE

It is true that the kinetic energy (KE) of molecules (not any substane) of a gas depends only on temperature, i.e. KE = (3/2)kT

The above expression gives the KE of a molecules at temperature T, But KE can generally be expressed, in classical mechanics, equal to (1/2)mv^2, thus we have (1/2)mv^2 = (3/2)kT

That is to say, the temperature determines the amount of KE possessed by a gas molecules, which is the same regardless of the kind of gas. But the speed (r.m.s. speed) of molecules of a "heavy" gas is slower than that of a "light" gas.Hence, the term (1/2)mv^2 gives you an indication on the molecular speed, which is NOT the same for all gases under the same temperature.

In your example of oxygen and hydrogen, you can conclude that hydrogen molecules are having speed much faster than oxygen molecules do if they are at the same temperature, due to the lighter mass of hydrogen molecules. But at any temperature, the average KE of molecules in both gases must be the same.

This is just expressing the KE in terms of different physical quantities.

These are the different views from microscopic and macroscopic.