The structures of separation on a smooth circular cylinder immersed in a sinusoidally oscillating flow are examined in detail for a constant value of the frequency parameter $\beta $ as the Carpenter–Keulegan number $K$ is systematically increased from the marginally stable to fully separated region in the ($K$, $\beta $)-plane. The positions of the separation points are measured using sublayer fences, flush-mounted hot-film sensors, and extensive high-speed video recordings. The variations of the length scales of the resulting structures, the irregularity of the dye concentration fields, and measurements with two sensors have shown that the separation is three-dimensional, time-dependent, often turbulent, and far from being an eruption of a double-sided single shear layer, or a self-contained bubble. The increase of the three-dimensionality of the flow, evolution of various sizes of structures, secondary separations within the primary separation zone, and the occasional eruption of multiple shear layers are quite similar to the first direct numerical simulation of a laminar separation bubble in the presence of an oscillating inlet flow.