"Oxfam estimates that the richest 85 people in the world own half of all wealth." This is not what Oxfam's estimates say, it contradicts them. Here is the original quote: "The bottom half of the world’s population owns the same as the richest 85 people in the world."
What point are you trying to make? Those two statements are the same.
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.