To measure a double wall oven that is already installed, turn off the power needs at the circuit breaker, remove any cabinet trim, and remove the screws securing the oven to the wall. You can pull the oven out a couple of inches and measure around the opening.
Before sliding out the wall oven, measure the overall dimensions of the front of the oven. With the oven door open, remove the screws securing the oven to the wall. To avoid damaging the cabinetry, it is important to remove any trim.
Without pulling on the oven door, pull the oven a couple of inches away from the wall, since any more than a few inches may cause the oven to fall out. With the oven slightly away from the wall, use a tape measure to measure the opening along the top, bottom and sides. If possible, it is also helpful to measure the depth of opening from the front edge to the back wall, although most modern wall ovens have a standard cabinet depth of 24 inches.
Some appliance manufacturers list details for old models on their websites. Before measuring a wall oven that is already installed, it may be helpful to check online for the dimensions.