An oven appliance defines a vertical direction, a lateral direction and a transverse direction. The vertical, lateral and transverse directions are mutually perpendicular. The oven appliance includes a cabinet extending between a first side portion and a second side portion along the lateral direction. The cabinet also extends between a top portion and a bottom portion along the vertical direction. The cabinet defines an upper cooking chamber positioned adjacent the top portion of the cabinet and a lower cooking chamber positioned adjacent the lower portion of the cabinet. The oven appliance also includes a single heat source in thermal communication with an ambient environment around the oven appliance by natural convection and a fan operable to provide direct thermal communication from the single heat to one or both of the upper cooking chamber and the lower cooking chamber by forced convection.