You are permitted 1 open jaw on a oneworld award, but you must return to the originating city. I have scheduled DCA-LGA-YYZ-LHR-CAI open jaw LHR-DUB-BWI (BWI/DCA/IAD are co-terminals, so OK)
It is 2 transits and 1 stopover per city. I had to spend a day in London one time since I had already used my 2 transits. Date/time changes free only for biz/FC awards, otherwise $100 change fee.
Also be aware that once the itinerary is ticketed, no changes to routing or CARRIER is allowed. I tried one time to switch an IB MAD-MIA to AA and was denied.