The oscillation of horseshoe vortex systems around the base of wall mounted obstacles in the transition range is considered. Existing experimental data from a variety of sources is reanalyzed in the light of two hypotheses—that vortex oscillations are caused by (a) oscillations of the entire vortex system, and (b) by vortex core instabilities. It is shown that both types of oscillation exist, and the regions in parameter space in which each exist are determined. The interaction between the different types of oscillation is also described.