We analyze the performance of bulk heterojunction organic solar cells under oblique incidence of light. In this regard, we present an optoelectronic analytical model that describes the current-voltage characteristics of bulk heterojunction organic solar cells taking into account the effect of angle of incidence. A closed-form general expression is derived for the optical generation rate under oblique incidence employing transfer matrix formalism. The resulting expression is then incorporated in the classical drift-diffusion transport and continuity equations of charge carriers to derive a unified expression of voltage dependent current density combining optical and electrical parameters. Thus, the model is capable of determining the accurate optical absorption in the active layer for varying angles of incidence as well as predicting the corresponding wavelength dependent external quantum efficiency of the device. The results are verified by comparing with published numerical and experimental results. We show that the maximum efficiency might be achieved at an oblique angle of incidence rather than normal incidence for certain active layer thicknesses. We also report the optimum angles at which the maximum efficiency occurs and show that they are active layer thickness dependent.