Using the density-functional theory calculations, we have investigated the effect of an in-plane electric field on the electronic properties and the structural stability of the edge-oxygenated armchair graphene nanoribbons (AGNRs). It has been found that the metallic-semiconducting transition and band gap modulation can be realized if a proper in-plane electric field is applied across the edge-oxygenated AGNR. Moreover, the critical strength of the applied electric field for the transition from metallic to semiconducting phase decreases with the width of the AGNR, while the range of the applied electric field required to maintain the semiconducting phase is increased when the width of AGNR decreases.