Um... Explosive
Climbers need to wear oxygen masks to help them breathe as the air is thin higher up Mount Everest than at sea level.
Rockets need to carry a steady supply of oxygen so that the fire that takes them off of the ground can burn. You must remember that to have fire you must have oxygen.
Question not clear.
Because your cells need oxygen. Blood has Hemoglobins inside it, which carry oxygen.
There is no free oxygen in space, so they need to carry oxygen with them to allow their fuel to burn and, in the case of manned spacecraft, for the crew to breathe.
Haemoglobin.
Oxygen
Plants need sunshine to carry out photosynthesis.
Gills carry oxygen fish need directly from the water
the process of exchanging gases
If you mean 'Why do climbers carry oxygen cylinders on mountains' then that is to survive. On high mountains like Mount Everest the air (oxygen) is so thin you need help to breathe, so they use oxygen from a cylinder.
Organisms use the oxygen they absorb from the air for their cells. The cells need this oxygen in order to carry out their functions.