If there is no oxygen on Pandora then how did the Jake make fire to fight with the animals?
Like Earth, Pandora has a nitrogen-oxygen based atmosphere. However, other gasses in the atmosphere make it inhospitable to humans. Pandora's air contains over 18% more carbon dioxide, as well as a concentration of hydrogen sulfide; both make the air unsuitable for humans. There is also a 5.5% concentration of xenon in the atmosphere, a heavy gas which contributes to Pandora's air being about 20% denser than Earth's. Humans out on the surface make use of an exopack which filters out the toxic gasses. It is apparent the Pandora has oxygen (flame burns yellow: oxidants other than oxygen would not burn yellow); plants are green (redox transport ligands other than magnesium would not result in green plants); Pandora has water. A toxic atmosphere with sulfides and heavy inert gases would make it unbreathable. However, in the scene with Jake gasping for breath, a toxic atmosphere would have him coughing out not gasping in. Perhaps there is enough xenon to displace oxygen in the lungs, but there is no experimental evidence to support this. Perhaps we need to fund a study. I will not offer myself as an experimental subject, however.