We report first-principles transport calculations in chemically functionalized graphene nanoribbons. The effect of the joint attachment of hydroxyl and hydrogen groups on the graphene surface is investigated as a function of defect location and coverage density. The chemical bonding of a single defect pair (C−OH and C−H) is shown to considerably alter the conduction capability of ribbon channels, similarly to an sp³ type of defect. With transport calculations in disordered ribbons with lengths up to the micrometer scale, the elastic mean free paths and conduction regimes are analyzed. Even in the low grafting density limit, transport properties are found to be severely damaged by the functionalization, indicating a strong tendency toward an insulating regime.