Explain why a balloon filled with O2 at room temperature collapses when cooled in liquid nitrogen where as balloon filled with He does not

When cooled by liquid nitrogen, the balloon shrinks (not as much as the air-filled balloon) and it sinks down on the table. When heating up, the balloon slowly rises and flies up in the air again. Explanation 1: The volume of the balloon decreases by the low temperature, because the gas inside is cooled down.