What is causing the increase in temperature? more oxygen or more carbon dioxide

Carbon dioxide is the cause of higher temperatures all around the world also known as global warming

more carbon dioxide

Explanation

heard of greenhouse effect right?

the core factor responsible for increase in temperature is definitely carbon dioxide: the ozone depletion leads to the creation of ozone whole as a result of which the heat moving from the earth (infra red rays) to the atmosphere gets reflected back and intensifies the temperature