An analytical method is developed that can be applied to a large variety of optomechanical systems to study entanglement between two subsystems of interest. The method is based on a system parameter that can be considered as perturbation parameter. It is shown that the method allows researchers to draw both qualitative and quantitative conclusions about the perturbation parameter at hand. First, the conclusion can be drawn whether or not the parameter when scaled up slightly induces entanglement between the subsystems. Second, physical insights into the role of model parameters for the emergence of entanglement can be obtained based on the perturbation theoretical analytical expressions. Third, quantitative predictions of numerical simulations that so far dominate the literature in the field of optomechanical entanglement can be validated at least in the limit of the vanishing perturbation parameter.