From Longman Dictionary of Contemporary Englishoxygen maskˈoxygen mask noun [countable] 
THBHa piece of equipment that fits over someone’s mouth and nose to provide them with oxygen
THBHa piece of equipment that fits over someone’s mouth and nose to provide them with oxygenExamples from the Corpus
oxygen mask• They placed an oxygen mask on my love and dared me to count up to ten.• They coaxed everything from pygmy mice to snakes to cheetahs into running on a treadmill while wearing an oxygen mask.• Reynard wears an oxygen mask and breathes pure air.• The shock was terrific, he gasped into his oxygen mask, his hands clenching involuntarily.• He remembered his oxygen mask and placed it over his mouth after removing the smog mask.• But he saw nothing, nothing but dark sky: The oxygen mask was filled with sweat.• He told me that the oxygen mask had pulled away from my face some, and that I was probably just blacking out.• It felt good sitting there with the oxygen mask off and not having to worry about flak. 