He did not patent it though, the first patent went to Swedish inventor J.O. Lundberg in 1896. The o-ring did not make it to North America until 1937 when Niels Christensen filed a patent for it.
The US government found the mighty o-ring to be quite usefull, so they took the rights to it and paid Christensen $75,000.00 back in WW2 (not sure of actual date). They simply took it and gave license to various companies to make them as they were needed for the war effort.
This is when it began its life a general seal for many applications.
Now it is used in places you would not think of such as electronics, inside your watch, the space shuttle, as padding in between articles, as drive belts, and as rollers.