Gerard K O'Neill was a physicist; his claims to fame were inventing the colliding storage ring; and asking a good question.
Asking the right questions in life is important. His question was:
"Is the Earth's surface the best place for a technological civilisation?"
The answer of his class was no: In space there are easily tapped, large, reliable sources of free solar energy; and abundant sources of raw materials on the Moon, Mars' moons and the asteroids.
The difficult part is getting to space. (See costs of launching to orbit). Once you're there, you're a few months away from anywhere between Mars and Venus, and about 3 days from the moon. Fuel to do this travelling is likely to be available in most places if you look hard enough. Trade routes can be set up and colonies would be able to grow exponentially with no limits we are likely to hit in centuries.
However, many people considered the idea of living in a space station too confining and still do.
But, colonies can be huge. Gerard worked on grandiose designs such as Island 3 which is an O'Neill Colony 20+ kilometers in diameter and well within the capabilities of mankind. His book The High Frontier talks about these ideas.
Gerard tried to work with NASA and the US government to achieve his vision. In the end he was defeated mainly by the realities of the very poor economics of the Space Shuttle. Still, there is real hope that space tourism and travel may yet open up the High Frontier within our lifetimes, and his visionary view of the future may be realised.