There is NO mileage limit on the ONEWorld Explorer fare. There are segment limits,stopover limits and you pay according to the number of continents you pass through but there are NO mileage limits.
The maximum miles I could work out for an itinerary was around 90000. My trip is just over 80000 miles - a waste of 10000 mile earning opportunities I know but part of the reason is that American conspires against me by not flying to Alaska in winter!!