The quantitative flow structure and associated loading arising from stream wise oscillations of the cylinder in a steady current are investigated experimentally via a technique of high-image-density particle image velocimetry (PN) and simultaneous measurement of the instantaneous forces. This approach leads to a direct correlation between instantaneous patterns of vorticity in the near-wake and the unsteady loading on the cylinder. The order of occurrence of basic patterns of vorticity during an oscillation cycle of the cylinder determines the type of instantaneous signature and time-averaged spectrum of the transverse force. Furthermore, the effects of proximity to a free-surface are examined. Both the patterns of near-wake vorticity and the transverse force signatures, and thereby the possible occurrence of lock-on states of vortex formation, are markedly altered, relative to the case of a fully-submerged cylinder.