Proceed with caution. Can you put the microwave someplace else or use a microwave drawer? Even if you find a configuration now that will look appropriate above your wall oven, consider that in 10 years you may need to replace the microwave and the trim kit may not work with what is available.
We replaced an original built-in microwave about 15 years ago, and only found smaller units, but were able to find a trim kit that worked. Two years ago that microwave died, and we had to replace it with one that was even shorter. We barely made the new trim kit cover the original cutout, and that was with additional trim.

The trim kits are stupidly expensive. We did this with a tabletop microwave:

If you search “microwave shelf” in Houzz discussions you’ll find many similar to what darbuka shows. This seems like a far more pragmatic approach. Good luck with your kitchen!

I did something similar as what darbuka did and it worked out very well. And given how disposable a lot of appliances are these days, including microwaves, paying for any kind of trim kit would be a waste IMO.

Microwave trim kits are expensive. They just are. So what? Chose the one you like that fits the microwave you’ve chosen and move on. It’s a few hundred dollars of the overall renovation.

I'd consider the slim Whirlpool WMT50011KS whose door opens to the left, is only 14" high (30" wide), includes the trim kit, and is only $700.

You may be able to install it over the Whirlpool WOS51EC0HS wall oven for a uniform appearance.

Like what you did there @darbuka.

Thanks, @jrb451. Our contractor modified the shelf so it looks almost built-in, without the $200 trim kit…which was just $100 less than what the microwave cost! He left enough air space behind, on top, underneath, and on the sides of the microwave for heat to escape.