We have performed electron spin resonance (ESR) measurements on a large assembly of graphene oxide (GO) and reduced graphene oxide (RGO) flakes. In GO samples the Curie tail is coming from 1.4 x 10(18) cm(-3) of localized spins. Although reduction of GO was expected to reestablish the pristine properties of graphene, no Pauli-like contribution was detected and only a low concentration of 1.2 x 10(16) cm(-3) spin carrying defects were measured. Our study, completed by resistivity measurements, shows that the carrier transport in RGO samples is dominated by hopping. The incomplete reduction of GO leaves behind a large number of defects, presumably the majority of which are ESR silent, causing the Anderson localization of the electronic states. Slight doping with potassium indicates the appearance of a Pauli contribution in the spin susceptibility.