We investigated the fragmentation dynamics of highly excited states of molecular oxygen using femtosecond transient photoelectron spectroscopy. An extreme ultraviolet (XUV) pulse populates the autoionizing Rydberg series converging to ${\mathrm{O}}_{2}^{+}\phantom{\rule{4pt}{0ex}}c\phantom{\rule{0.16em}{0ex}}^{4}\mathrm{\ensuremath{\Sigma}}_{u}^{\ensuremath{-}}$, and a femtosecond near-infrared (IR) pulse was used to photoionize these states as they dissociate. Monitoring the differential photoelectron spectra as a function of time delay allowed us to obtain the relaxation lifetimes of these Rydberg states. We observed a photoelectron signal corresponding to the formation of a $4p$ excited atomic oxygen fragment, which is not an expected dissociation product of the $({\mathrm{O}}_{2}^{+}\phantom{\rule{4pt}{0ex}}c\phantom{\rule{0.16em}{0ex}}^{4}\mathrm{\ensuremath{\Sigma}}_{u}^{\ensuremath{-}})nl{\ensuremath{\sigma}}_{g}$ Rydberg series. Analysis of the time-dependent photoelectron spectra and photoionization calculations indicate that this fragment results from a previously unexplored $({\mathrm{O}}_{2}^{+}\phantom{\rule{4pt}{0ex}}^{4}\mathrm{\ensuremath{\Pi}}_{g})4p$ repulsive state and that, contrary to expectations, this multielectron excitation pathway presents a substantial cross section. Our study demonstrates that two-color time-resolved differential photoelectron spectroscopy is an excellent tool to study the fragmentation dynamics of such multielectron excited states, which are not easily probed by other means.