Oleum is double-strength sulphuric acid.
Also called Fuming sulphuric acid, Disulphuric acid, Nordhausen acid and Pyrosulphuric acid. Or, of course Fuming sulfuric acid, Disulfuric acid and Pyrosulfuric acid
This stuff is nasty. It has an oily texture (though you'd be crazy to dip your fingertips into the stuff). It is nominally colourless, though often has a yellow or brown colour from excess sulphur.
The name 'Fuming' comes from the fact that it will readily release SO3 which is another really nasty material, which appears as a kind of white smoke above the Oleum.
It is made in a similar process to sulfuric acid--in fact is the penultimate step in the production of sulfuric acid. Despite being extremely corrosive, it can be--and usually is--stored in glass bottles. However, it is rarely used in pure, concentrated form, but needs to be diluted with care.
And if you add water to the acid, it will do a lot of very nasty things: up to and including explode, sending liquid acid flying around the room. Nasty
The chemical formula is H2S2O7. Think about this for a while.
Sulfuric acid is H2SO4 Right?
Two lots of that makes H4S2O8. With me so far?
Subtract one lot of water (that's H2O)
... in effect it is double strength H2SO4. It craves* water. It will suck the water out of anything and once it has sucked the water out, will burn any bases in there transforming them into salts and water.
If the water is not readily available, then it will do chemical magic to suck hydrogen and oxygen atoms out of anything and use them to satiate its thirst*. Bear in mind that most proteins and organic matter have hydrogen and oxygen atoms attached to the carbon backbone.
Put some of this onto a piece of organic material and all you will be left with is a carbon foam. Ewwww!
* I know. Anthropomorphism is a Bad Thing in this context. We who use this in a figurative sense risk implying that mineral acids think or have feelings. They don't.
Yes, it *would* have been more accurate to say oleum has a very strong electro-chemical potential and that the SO3 unit is extremely reactive in the presence of hydrogen and oxygen, but heck, it seemed more descriptive to say it has a thirst for water. Sorry if I offended.