The effect of high-pressure oxygen (HPO) annealing on the electrical, magnetic, and magnetoelectric (ME) properties of room-temperature multiferroic BaSrCo2Fe11AlO22 ceramics was investigated. The electrical resistivity of the ceramics was found to strongly depend on the partial oxygen pressure during annealing at 1040 degrees C. Samples annealed under 10atm of oxygen exhibited a resistivity of up to 1.6x10(9)cm at room temperature, more than two orders of magnitude higher than that of samples without oxygen annealing. Thermally stimulated current and complex impedance measurements suggested that the enhancement of the resistivity by the HPO annealing originated from a decrease in the amount of defects related to oxygen vacancies and an increase in the resistance of grains and interfaces. HPO annealing also affects the magnetic-field response of spiral magnetic ordering, which is ascribed to the ME properties. Furthermore, samples subjected to HPO annealing exhibited a lower contribution of the space charges trapped at the grain boundaries and/or defects to the magnitude of the measured magnetoelectrically induced electric polarization P. The present results indicate that HPO annealing is an effective method to evaluate authentic ME effects in multiferroic BaSrCo2Fe11AlO22 ceramics.