I was wondering, what would happen if there was a large amount of oxygen released into Jupiter's atmosphere, than igniting it? Because Jupiter is 90% gas, therefore being a big fire waiting to happen (with air, since fire needs oxygen).
Unless the amount of oxygen is very large, not much would happen.
Certainly a volume of oxygen would when ignited vigorously burn in the mostly hydrogen atmosphere, but it would be a local phenomenon. To actually set "things on fire" requires introducing a comparable mass of oxygen to the hydrogen - literally planetary masses of oxygen.
How much? If we have 0.9 Jupiter masses of hydrogen, we need 0.9*(16/2)=7.2 Jupiter masses of oxygen to make the reaction perfect (the 16/2 is the ratio of oxygen atom mass to two hydrogen atom masses). That sounds impractical.
What happens if Jupiter is ignited?
Essentially that happened before Jupiter formed. Jupiter is a bit less than 75% hydrogen and a bit less than 24% helium. That adds up to a bit less than 99%. The rest is trace elements, most of which readily combine chemically combine with hydrogen to form compounds such as methane, ammonia, hydrogen sulfide and water.
The hydrogen in Jupiter that remains as hydrogen as opposed to the hydrogen compounds mentioned above does so because there's nothing left in Jupiter with which that hydrogen can combine chemically. Light a match inside Jupiter and nothing much will happen because the chemical reactions that can occur did occur 4.6 billion years ago. The previously mentioned hydrogen compounds in Jupiter (methane, ammonia, hydrogen sulfide and water) mostly formed before Jupiter was even Jupiter.