Darth Vader's OOT Origin of the injuries:
http://www.theforce.net/swtc/injuries.html
The second draft of The Empire Strikes Back screenplay tells an alternative story, indicating that Vader fell into a nuclear reactor shaft rather than lava. In this case, his infirmities may have something to do with mutation, radiation sickness and extensive cancer.
Update 4: Edited to make shorter
Note that Lucas had to change main things of the Vader that We know in the OOT to make his Revenge of the Sith's vader.
http://www.secrethistoryofstarwars.com/visualdevelopmentofdarthvader.html
http://www.theforce.net/swtc/injuries.html#skeleton
note that OOT's Vader has eyebrows, but in the '04 SE dvds, Lucas had the eyebrows removed and also In the '04 SE dvds, added more burn scaring to vader's face.
I once saw a story inwhich Lawrence Kasdan was talking about the unmasked vader and said that Vader was a Mutant. I can not find the story on the web and I wondering if someone remembers it.
Here is a sketch of now an unmasked Vader from ESB: http://secrethistoryofstarwars.com/unmaskdrawing.jpg
From all that I have learn it seems that the Vader from ESB was indeed a radiated mutated person.
In the Return of the Jedi novel, Anakin fell into a molten pit, inwhich Anakin's back is burned. In the Return of the Jedi novel, only Anakin's back is said to burn and no details of other places.
Here in the classic back of unmasked vader's head scene from ESB, you can see the burn scaring up the back of his head: http://www.theforce.net/swtc/Pix/dvd/ep5/ldvader2.jpg
but, also in the picture you can see that vader's head has more damage than burn scars.
From all of this , we can put together that the radiated mutated Vader from ESB had been burn up the back, like the vader in the Return of the Jedi novel.
It seem the "lava burns only" vader came sometime after the OOT was made and that the OOT vader's injuries are not made from lava burns only.
So.... in the End, It seems that the vader in OOT was both a radiated mutated person from falling into a nuclear reactor shaft and also had lava burns up his back from falling into a molten.
http://darthvader.blog.dada.net/archive/images/DarthVader.jpg
One of the best pictures of The old radiated mutated Burned Darth Vader from OOT
R.I.P (1976-1984)