Well you can take a candle, light it and cover it with a glass, You'll notice that the candle will stop lighting. As we know that oxygen is needed for burning, and the glass stopped the supply of air which has oxygen needed for burning. 
This shows that their is oxygen in air. (:
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the experiment of atmospheric oxygen is required for burning...

1. take a potted plant

2.cover few leafs with a plastic

you will notice that the leave has become dead in lack of oxgen

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