This study experimentally investigates the effect of channel blockage on the wake structure of a rotationally oscillating cylinder. The circular cylinder is placed symmetrically inside the channel. The Reynolds number based on constant upstream channel inlet freestream velocity is fixed at 150, and wall spacings of two, four, and six cylinder diameters are used. Cylinder oscillation amplitudes and normalized forcing frequencies also vary along with the gap distance. The diagnostics are done using laser-induced-fluorescence flow visualization. It is observed that rotary oscillations induce vortex-street formation at channel width of two cylinder diameter where there is no shedding in unforced case. The wake structure is found to resemble the wake structure in isolated cylinder case at a wall spacing width of six cylinder diameter.