The O'Neill cylinder would be hollow, containing a very pleasantly Earth-like environment: a breathable atmosphere, hills, cities, rivers and lakes, forests, picnic territories.
The L5 Society, named after L5, one of the five Lagrange points in the Earth-Moon system, favoured by O'Neill as a stable location for his great stations, was founded in the 1970s to propagandize for a government-funded construction campaign. Energy independence was the key for the L5 society--solar-cell arrays assembled by the colonies would be transported to varius geosynchronous orbits and beam the power that they generated down to Earth in the form of microwaves. Providing both an impressive presence in space and the key to a world running on cheap clean energy, this grand projet would be one for the ages. The decade's space colony artwork portrayed an archipelago of many small worlds, all of them Earth-like, all of them eminently comfortable in the best High Modernist fashion.
Slashdot reports that someone writing in a United States military journal favours a preemptive seizure of the Lagrange points (thanks, Will), to prevent these regions from being occupied by aggressive foreign powers. That bit of preemptive astropolitics, alas, seems to be the only survival of the O'Neill-L5 grand projet to the present. Back in the 1970s, it was thought that space travel could be cheap and that it was a trivial task to establish a viable self-contained biosphere. Both of these assumptions turned out to be quite false. The United States just wasn't interested in space travel after Apollo, and space colonization was much too costly.
And yet. Last Thursday I wrote about the resettlement of northern Bohemia after the expulsion of the Sudeten Germans in 1945, and how this resettlement was used to create a region strongly attached to production for production's own sake, and that this sort of tendency was common to Communist and non-Communist states. So, I posted a WI on SHWI: WI Soviet O'Neill colonies?.