Our 2010 over the range microwave oven is about it die. It wents outside through an opening in the wall behind the microwave. There's a port opening when the fan starts and it blows out through there. Are there any standards as to where and how big the outside vent is across microwave ovens or am I most likely to find one of the same brand (in this case Whirlpool)? Or is this not standard even within oven brands so it will be hard to find an oven that fits at all?
Most appliance manufacturers provide cad drawings that show the external mating features of their devices.
You will have to do a little bit of digging but you should be able to find the details of the venting.
I have installed 6 otrs.
The size of the opening is fairly standard but the placement varies. The width of the otrs vary but the opening is typically horizontally centered in the back or top of the unit (some have the option to choose).
The door of the otrs also vary - some have zero clearance hinges which allow tight placement with side cabinets and some have doors/hinges that require some amount of clearance or will bind with the side cabinet doors.
Your install is the hardest as everything has to line up perfect to re-use the appliance size and hole position - good luck!