I've never had one that provided adequate ventilation even the ones that had an outside duct. That said I have one now because I have no exterior duct work, it would be difficult to add the duct work and I think OTR is an efficient location for a microwave assuming there's a reason you can't have a hood that provides decent ventilation. I would not have an OTR microwave if there was another reasonable alternative and I'm not a ventilation zealot by any means. When we remodeled our previous kitchen I was perfectly satisfied with a 300 CFM ventahood (didn't want to install makeup air required for anything larger) and used it as little as possible as fan noise drives me crazy.

Thanks kaseki and 3katz4me. Your input is very helpful and seems to verify what I was told by an appliance store salesperson. I live in a townhome and am not even sure yet if the HOA will allow me to vent either OTR micro or Hood as it would have to vent to facade of our homes. However still wanted to verify with experienced users.

I live in a townhouse also, and was unable to vent to the facade. The code here is your vent cannot be within 10' of an adjacent house, or 5' of any operable window. (which makes perfect sense.) That condition does not exist on the front of the house. So I ended up with a recirculating OTR microwave. I will be interested if you are allowed to do it and what you end up with. There are certainly some houses around here with vents that don't meet the strict criteria. Code interpretation and enforcement is definitely variable.

The OTR microwave is pretty useless as ventilation. I typically open a window. Given the restrictions, I'm still happy with it since I couldn't vent outside anyway.

Adding to what Kas and 3katz4me said, the U.S. Department of Energy and the EPA have been sponsoring studies of kitchen ventilation and indoor air quality. Some of these studies compare venting options including OTR/MWs. If interested, you can find some of the recent studies by googling "researchgate + kitchen vent."

DIY2Much2Do commented that "Code interpretation and enforcement is definitely variable." The same is true of HOAs and condo agreements. Some code enforcers and some HOA may see some discretion in applying the rules. Some will not. Won't know until you try. Let us know how it works out for you.

There are recirculating standard hoods available, from at least Broan and Vent-a-Hood. These use stacked filters to remove grease and odor. While not as effective as complete removal of effluent to the outside, they have been noted in other threads here as definitely better than exhausted OTR microwave venting or recirculating versions of OTR microwave venting.

It's not only code and the HOA. Where I live, for example, the hurricane master insurance policy holder for the complex takes a very dim view of putting additional holes in the envelope.

They make lots of noise. Venting outside is better than recirculating the air. That said they do not compare to a real hood vent.

so much good information! Thank you all.

We have a condo in FL with the same issue. They would never let you put a hole in the building and vent outside. I rarely turn the OTR MW fan on except when I cook bacon. Even then, I open a window.