The understanding of magnetoresistance (MR) in organic spin valves (OSVs) based on molecular semiconductors is still incomplete after its demonstration more than a decade ago. Although carrier concentration may play an essential role in spin transport in these devices, direct experimental evidence of its importance is lacking. We probed the role of the charge carrier concentration by studying the interplay between MR and multilevel resistive switching in OSVs. The present work demonstrates that all salient features of these devices, particularly the intimate correlation between MR and resistance, can be accounted for by the impurity band model, based on oxygen migration. Finally, we highlight the critical importance of the carrier concentration in determining spin transport and MR in OSVs and the role of interface-mediated oxygen migration in controlling the OSV response.