That is absolutely not the same. One is referring to half the wealth in the world, one is referring to the wealth owned by the poorest half of the world (which is strictly less than half by definition, and in this case far less than half).
Doesn't make the fact not-disgusting, just slightly less so.
They are not. The bottom 50% of people owns a fraction much smaller than 1/2 of all available wealth.
What Oxfam is saying is: if you rank people by wealth on the x axis then plot their wealth of the y axis, the integral (sum, or area under the curve) of the function on the first half of the graph is equal to the integral on the last 85 points. Try actually drawing it, it might help you understand.
The bottom half of the World's population not only don't have a representative half of the World's wealth, they basically own diddly squat compared to their numbers.
Doesn't make the fact not-disgusting, just slightly less so.
EDIT: Wow, 5 corrections in two minutes.