The dynamics of electronic autoionization in ${\mathrm{O}}_{2}$ has been studied using a new apparatus which combines a free-jet supersonic expansion with synchrotron radiation. Ions and electrons were analyzed by a double time-of-flight spectrometer. The spin-orbit sublevels of the $^{3}\mathrm{\ensuremath{\Pi}}_{\mathrm{u}}$ (v=0 and 2) Rydberg states in ${\mathrm{O}}_{2}$ were selectively excited and the resulting ${\mathrm{O}}_{2}^{+}$ final states were determined by time-of-flight photoelectron spectros copy. A strong variation of the $^{2}\mathrm{\ensuremath{\Pi}}_{1/2\mathrm{}\mathit{g}}$${:}^{2}$${\mathrm{\ensuremath{\Pi}}}_{3/2\mathrm{}\mathit{g}}$ branching ratio was observed. This variation results from the selection of a single continuum wave function in the autoionization process.