Mileage is actual miles point to point. So 67 miles MKE-ORD and 5287 miles DFW-EZE. You can fly SCL-SYD (on LA) and a 35K Oneworld award would cover your routing.
As for buying a OWE, it would be 5 continents if you want to visit Australia and South America (which would seem to be your requirements). You have to transit Europe, Asia and North America ... so that is 5 continents. You do get the 4 segments in each continent, though -- back to the drawing board?