What route was this? I believe BA's rules apply LHR-MAD as that's technically a BA flight that happens to be operated by Iberia (presumably so they can operate as one airline out of T5 without the need for the check-in staff to learn Iberia's systems). On other routes then presumably you'd get an extra bag as currently the oneworld rules only exclude BA HBO fares.

Note: in Madrid BA and IB operate as separate airlines. So BA's rules will apply on BA flights and IB rules on IB flights.