[1] He was a Robert R. McCormick Fellow at the Enrico Fermi Institute from 1988 to 1991, before becoming an assistant professor at the University of Chicago.
[1] Ong's early research focused on electron–positron annihilation at SLAC PEP, including development of the first vertex drift chamber for a collider experiment with Martin Perl, followed by his doctoral work on the B hadron lifetime.
[2] His research interests shifted to gamma-ray astronomy in 1990 after joining the Chicago Air Shower Array experiment, with the encouragement of James Cronin.
[3] He has since continued research in astrophysics with involvement in STACEE, the Fermi Gamma-ray Space Telescope, VERITAS, GAPS, and the Cherenkov Telescope Array (CTA).
In 2013, he was elected a Fellow of the American Physical Society "for his contribution to high energy particle astrophysics, in particular his contribution to very high energy gamma ray astronomy, where his research has spanned four decades of the electromagnetic spectrum.