This paper calls Mars uniquely suitable, but only briefly references O'Neill on free-space habitats built out of lunar or asteroid material. His main argument a

This paper calls Mars uniquely suitable, but only briefly references O'Neill on free-space habitats built out of lunar or asteroid material. His main argument against seems to be the need to protect from radiation, but it's not like O'Neill and successors did not know -- they had designs for that. Yes, a habitat with gravity and shielding and agriculture would be expensive. So would colonizing Mars.