A combination oven that is operable with convection air, impingement air and microwave energy in various combinations thereof. The oven has an oven chamber and a fan box that are located front to back. A fan in the fan box circulates heated air by discharging via openings in a top and a bottom and taking in via an intermediate opening of a baffle plate. Impingement plates are easily installed and removed in the oven chamber to provide impingement air upwardly or downwardly. At least one of the impingement plates is installed and removed by a sliding motion. Microwave energy is provided through the side walls of the oven chamber. Intake ports for cooling air are located in a bevel between the side walls and bottom wall of the oven's outer enclosure so as to allow the oven to be located right next to other structures, such as a wall. An interlock assembly is also provided for the oven door.