The electronic and magnetic properties of varying width, oxygen-functionalized armchair graphene nanoribbons (AGNRs) are investigated using first-principles density functional theory (DFT). Our study shows that O-passivation results in a rich geometrical environment which in turn determines the electronic and magnetic properties of the AGNR. For planar systems, a degenerate magnetic ground state, arising from emptying of O lone-pair electrons, is reported. DFT predicts ribbons with ferromagnetic coupling to be metallic, whereas antiferromagnetically coupled ribbons present three band gap families: one metallic and two semiconducting. Unlike hydrogen-functionalized AGNRs, the oxygen-functionalized ribbons can attain a lower energy configuration by adopting a nonplanar geometry. The nonplanar structures are nonmagnetic and show three semiconducting families of band gap behavior. Quasiparticle corrections to the DFT results predict a widening of the band gaps for all planar and nonplanar semiconducting systems. This suggests that oxygen functionalization could be used to manipulate the electronic structures of AGNRs. © 2013 American Physical Society.