Viscosity Solutions and American Option Pricing in a Stochastic   Volatility Model of the Ornstein-Uhlenbeck Type
  In this paper, we study the valuation of American type derivatives in the stochastic volatility model of Barndorff-Nielsen and Shephard (2001). We characterize the value of such derivatives as the unique viscosity solution of an integral-partial differential equation when the payoff function satisfies a Lipschitz condition. 
Alexandre F. Roch