As said above, you cannot add this to a BA reward booking. However, AA policy is that they will rebook you if the inbound OneWorld flight causes you to miss an AA flight even when they are on separate bookings.
I wonder if this policy has changed a little. I thought AA said they would reroute you regardless of the fault/cause but has the text changed?
http://www.aa.com/i18n/agency/Bookin...tkt_policy.jsp
AA to/from AA or a oneworldŽ Carrier
If a customer is holding separate tickets on AA or another oneworld carrier, customers holding separate tickets where travel is on oneworld airlines should be treated as through ticketed passengers. In the event of a disruption on the originating ticket, the carrier responsible for the disruption will be required to reroute the customer to their final destination. The ticket stock of the second ticket must be of a oneworld carrier, eligible under the Endorsement Waiver Agreement.
If it's AA->BA and AA cause the misconnect this policy would protect you. But I'm not sure an AA policy will protect you if you misconnect BA->AA and AA put the responsibility on BA: surely that would depend on BA's policy not AA's.