Single-crystalline Cr-doped In2O3-delta nanostructures with diverse morphologies including nanotowers, nanowires, and octahedrons are synthesized by using a vapor transport method. X-ray photoelectron spectroscopy results indicate that the as-grown samples contain 3 at. % Cr and are significantly oxygen deficient. The large surface-to-volume ratio in the nanostructures enhances their susceptibility to the postsynthesis treatments; high-temperature annealing in air boosts the oxygen contents in the samples, which is accompanied by a weakened defect-related emission in the photoluminescence spectra. Magnetization measurements on the as-grown and the annealed nanostructures suggest room-temperature ferromagnetism, and importantly the ferromagnetism is stronger in samples with higher oxygen deficiency. Electronic band alterations as a result of the Cr doping and the oxygen vacancies as well as the formation of bound magnetic polarons are suggested to play important roles in stabilizing the long-range ferromagnetism.