In collaboration with Burton Richter, O'Neill performed the first colliding beam physics experiment in 1965. In this experiment, particle beams from the Stanford Linear Accelerator were collected in his storage rings and then directed to collide at an energy of 600 MeV. At the time, this was the highest energy involved in a particle collision. The results proved that the charge of an electron is contained in a volume less than 100 attometers across. O'Neill considered his device to be capable of only seconds of storage, but, by creating an even stronger vacuum, others were able to increase this to hours. In 1979, he, with physicist David C. Cheng, wrote the graduate-level textbook "Elementary Particle Physics: An Introduction". He retired from teaching in 1985, but remained associated with Princeton as "professor emeritus" until his death.