Characterisation of the unsteady hydrodynamic loads is essential for accurate predictions of the fatigue life and ultimate loads of tidal turbine blades. This paper analyses a set of experimental tests of the hydrodynamic blade root out-of-plane bending moment response to planar oscillatory motion, chosen as an idealised representation of the unsteadiness imparted by waves and turbulence. Phenomena associated with dynamic stall are observed which are sensitive to the oscillatory frequency and velocity amplitude. Flow separation is shown to result in loads significantly greater in magnitude than that for steady flow. Following flow reattachment, the load cycles compare relatively well with Theodorsen's theory for a two-dimensional foil oscillating in heave, suggesting that circulation due to the shed wake dominates the unsteadiness in phase with acceleration, over added mass effects. For attached flow, the effect of unsteadiness is comparatively much smaller. At low frequencies a phase lead over the velocity is observed, compared to a lag at higher frequencies. Multiple frequency oscillations are also briefly considered. Reconstruction of the multi-frequency response using both the steady flow measurements, and the single frequency measured response, is shown to offer a relatively good fit when the flow is attached, for lower frequency combinations.