From 1969 to 1971, O’Leary was assistant professor of astronomy at Cornell University, in Ithaca, New York; from 1970 to 1971.
He was a senior consultant at the NASA Ames Research Center; from 1976 to 1981, and a member of the research faculty for the Department of Physics at Princeton University in New Jersey; from 1982 to 1987
Brian O'Leary was a senior scientist at Science Applications International Corporation in Hermosa Beach, California.
NASA selected O’Leary as a scientist-astronaut by NASA in August of 1967. After completing a Ph.D. thesis on the physical properties of the Martian surface, NASA selected O'Leary specifically selected for a potential manned Mars mission when it was still in NASA's program plan.
Unfortunately, when Congress slashed NASA's budget in 1968, they had to cancel the program. O'Leary resigned from the astronaut program because of lack of prospects for a space flight.
O' Leary authored or co-authored more than one hundred peer-reviewed scientific papers, including:"The influence of lunar mascons on its dynamical figure," Nature, 220, 1309, 1968; and O'Leary, Campbell and Sagan, "Lunar and planetary mass concentrations,"Science,165, 651, 1969. Has also been an editor for several books, including, Space Manufacturing from Nonterrestrial Materials, edited by O'Neill and O'Leary, vol. 57 of Progress in Astronautics and Aeronautics, AIAA, 1977; and Space Resources and Space Settlements, edited by O'Neill, Billingham, Gilbreath and O'Leary, NASA SP-428, 1979. He has also authored more than ten books, including: Project Space Station, Stackpole Books, 1983; and Mars 1999, Stackpole Books, 1987.