Re: why is oxygen important to fire?

Good question..oxygen is very reactive and will combine with many things 
to make compounds.
The bonds break quite easily and it makes stable products, so the reaction 
gives out heat and light (exothermic).
As you say it is quite common - 21% of the air - so normally when we talk 
about burning we mean "in air" and so it is a reaction with oxygen.
However, other things do burn in other gases - sodium burns in chlorine to 
make sodium chloride (salt).
The same for other reactive metals like aluminium and some non-metals like 
sulphur.
Did you know that magnesium burns in carbon dioxide? It takes the oxygen 
from the carbon dioxide and makes magnesium oxide, leaving carbon!
See this link  http://www.madsci.org/posts/archives/may98/892337000.Ch.r.html