Wanderer:
The way to avoid buying a 5 continents fare and get away with 4 continents fare is to simply buy a cheap roundtrip MIA-CCS-MIA or JFK-CCS-JFK. Cost around $600 which is a lot cheaper than paying for an extra continent on Oneworld unless you have heaps of time and wanna earn a pile of extra miles.
On US domestic flights it might be an idea to arrange your routing with AA double mileage promos like ORD-DFW or ORD-Memphis-DFW(triple miles) or BOS/JFK to LAX/SFO and some others.