The torch burns quicker. A lot quicker, but you won't get a violent explosion. Fire requires three things - heat, fuel, and oxygen. You've increased the oxygen, but you haven't increase the fuel, so all will happen is that the torch will burn out far faster - if it lasts for 10 minutes, it might not even last for one. The oxygen won't ignite itself. Something like 2:1 hydrogen gas to oxygen, on the other side, would be far deadlier, but that's not what's in the room.

That's what happens to the torch. The adventurer gets oxygen poisoning because the 100% oxygen gas is not at low pressure and will probably die from a seizure if he stays in the room. EDIT: To clarify, the oxygen poisoning will occur within five minutes of breathing 100% oxygen at 1.0 amu for five minutes, death will only occur if you stay in the room for longer. The oxygen poisoning will wear off by breathing normal air.

Regarding Oxygen toxicity

This is rather an extended comment with some calculations than an actual answer.

I'd like to back up the matter with some actual calculations. I presume you mean a $20ft times 20ft times 20ft$ room when you say "20ft cube room".

That means we have a volume of $V=226.5m^3$. Let our initial pressure be at $p=p_0approx 100kPa$. Let $j$ be the volume flow per second of oxygen entering the chamber. From the ideal gas law we can then estimate how fast we will reach a critical level of oxygen pressure, as was linked in other answers and is more than 4 times atmospheric pressure.

$$fracp(t)Vn(t)RT=fracp_0Vn_0RT$$

where $n(t)=fracjcdotrhoMcdot t$

with $rho=1.429 kg/m^3$, $M=0.032kg/mol$ and of course $t=5min=300s$ and $p(300s)=4p_0$. We can now plug everything in.

We can calculate $n_0=fracp_0VRT=frac100kPa226.5m^38.314J/(K mol)293K=9298mol$

$j=frac4Mn_0rho t=frac4cdot 0.032kg/mol cdot 9298mol1.429kg/m^3cdot 300s = 2.78m^3/s$

So our pump will need to put at least 2780 litres of oxygen per second in the room at a constant rate in order to reach a potentially deadly level of oxygen just by the end of the 5 minutes.

The torch will burn much faster & hotter. There's a fairly common real-world example of this: the oxyactylene torch used for welding and cutting ferrous metals. Light it with just the actylene turned on, and you get a rather cool yellow flame that smokes a lot (rather like your adventurer's torch). Increase the flow of oxygen, and it burns with a hot, blue-white flame. https://en.wikipedia.org/wiki/Oxy-fuel_welding_and_cutting

Nothing of Note, Sorry.

Thus far almost everyone has been mesmerised by the Red Herring of oxygen toxicity. Given the time constraint of the query and given the chemistry of the experiment, I'm sorry to say that every answer, including the accepted, that involves toxicity is going to fail a reality check.

Rationales:

1. Let us not forget that Kleeber the Klobberer is bringing in a N2 soaked micro environment of his own. His own clothing, his hair, his bed roll, his barbarian hat, his spare clothing, the rope --- none of that is going to be instantly saturated with pure O2 inside of the time frame of the query.


2. Once he tumbles into the chamber, the N2 micro environment that surrounds him will create a kind of (temporary) shield. Once he realises it's dark and lights his torch, that N2 will serve as a buffer between the Rumelian candle of his torch and his own combustible gear. Surprise at the bright light and flame will most likely cause Kleeber to throw the torch across the room, thus giving him distance from the fire source as well as gaseous armour.


3. No one has yet taken into account the products of combustion. Once Kleeber enters the chamber, thus setting off the five minute timer, the (metabolic) combustion happening within his own body is going to pump CO2 into the atmosphere. Being an uncouth warrior hero, if he lets off a couple decent farts, he's going to pump quite a bit more CO2 and N2 into the atmosphere. Once he ignites the torch, the combustion of the oil, cloth and wood is going to rapidly emit a considerable amount of CO2 and H2O into the air. Wood emits the most CO2 of any common fuel, I've read 1900g of CO2 per 1000g of wood. A standard adventurer's torch weighs in at about 3 kg or so, which will make for a rapid infusion of nearly 6 kg of CO2.


4. Oxygen toxicity, as I've stated in other answers, is a thing. However, given the time frame of the query, it's just not of concern. It takes a while --- more than the five minutes allotted by the experiment --- to obtain relatively mild symptoms. Much more severe symptoms of course will occur as the time exposure is increased. Kleeber isn't going to be in this environment for a long enough time to suffer any serious reaction to the O2.

Conclusions:

• The initial conditions of the experiment, a pure O2 atmosphere, will be altered once the subject enters the chamber, bringing in outside & inert gasses such as CO2, N2 and even H2O plus traces of other atmospheric gasses.




• Fire in an O2 rich environment will release CO2 very rapidly, changing the chemistry of the atmosphere. Perhaps not enough to prevent oxygen toxicity over a long period of exposure, but sufficiently, perhaps, to keep our hero safe for the short duration of his exposure.




• As the wood of the torch is consumed and converted to CO2 & H2O, the subject will no longer be in a pure O2 environment. The wood will be consumed and the fire will self-extinguish. There will be little risk of clinically adverse oxygen poisoning within the given time frame.

A final consideration:

• Depending on the flow rate of the O2 pumps, Zappo's Thaumic Adignificator, might just get blown away on that 2780 l.p.s. gas flow without ever even lighting the torch! In that case, Kleeber won't risk possible combustion at all, and while the O2 levels will generally be higher for the duration of the experiment, his imported micro environment and the five minute limit will still keep him safe from oxygen poisoning.

A violent, rather sudden death.

When you are unsuspecting, you usually do not hold a torch with the burning end as far away as possible. You hold it in a comfortable position, so it will be maybe half a meter away from your head.

It's dark and unsuspecting Kleeber wants to see, so he summons that torch, in a normal, comfortable position.

The room was filled with 100% oxygen, and a pump keeps pumping extra oxygen into the room. That was happening during the, say, 5-10 seconds (probably more like 20-30 seconds!) while Kleeber was recovering from the fall, and getting up on his knees, too. So the room is not just filled with oxygen, it is filled with pressurized oxygen (... of unknown pressure). I'm neglecting the volume that Kleeber's body displaces, we can assume some oxygen went "out" when he came "in".

The torch, which is not very far away, will thus flash very brightly. How brigthly depends on the actual pressure and is pretty tough to tell, but let's just say it's worth a torch 5 times as big. That's probably not an awful exaggeration. No, it will not explode. But it will give off a considerable amount of very, very uncomfortable heat. Note that normally there's smoke and such (and flue gas is in principle burnable, only just there's normally not enough oxygen, except... think of the famous "Backdraft" movie). We don't have that here, it all burns up, there's plenty of oxygen.

So that will mean that not only Kleeber's sleeves (the leather parts will be only superficially oxygenized, but the cloth parts will be soaked) will catch fire, but also Kleeber's hair, which is a desaster. Not only because violently burning hair which wraps your head (possibly including respiratory paths) in a fireball is a definitely no-fun condition, but also because the sparks of burning hair will set everything else on fire, too.

There are documented cases of smokers who had such a funny surprise when lighting a cigarette while wearing an oxygenator. Now, that's a joke compared to what your poor Kleeber is experiencing.

High levels of oxygen, especially under pressure, can do miraculous things. Things that will not burn at all normally all of a sudden are not as reluctant to catching fire as you're used to.

There's documented cases where firefighters took an hour or two to take out a "petty fire" which they'd normally kill in seconds, solely because there was an open oxygen tube involved.

Reframing - the problem is not the torch, the problem is the oxygen.

Oxygen Toxicity

You've got an 100% oxygen environment, into which you're introducing "more and more" oxygen.

Kleeber's main problem is going to be oxygen toxicity. Lung damage, CNS damage, retinal damage.

You will also get some fairly spectacular fire, but that shouldn't be Kleeber's main concern, even if it's the most obvious.