Is there any advantage at all, let alone a huge one? When large DOF is desired you close down the aperture as much as you can: for a 35mm camera with 24-70/2.8 zoom on it that would be f/22, and it's exactly the same for a mft camera with a mounted 12-35/2.8. But f/22 on mft would result in a lower resolution than f/22 on a 35mm camera, with diffraction on mft eating some if not all of the difference in DOF. What's more, 35mm sensors are just better in general, more resolution and larger photo sites, so the image from 35mm will be superior anyway.

There are tests out there, for example Amateur Photographer did one some time ago, showing that in many (most?) cases shooting with the narrowest aperture doesn't make sense since whatever you earn in DOF you just lose due to diffraction, harming the whole image in the process. Macro is probably the only exception, and yet even with macro normal apertures and focus stacking are used as often as possible.

The image from a FF camera may well be superior, for a number of reasons, but you've missed the point of my (and other) posts. If f22 on a FF camera gives you the DOF you need, you wouldn't need to stop down to f22 to get the same DOF on m43. You'd only need to stop down to f11. Yes, 35mm at f11 on m43 will give the same DOF as 70mm at f22 on FF*. The idea that you "stop down as far as you can" displays a lack of knowledge and understanding.

* Approximately. It actually is more complex than simply using the crop factor, but it's close enough for discussion.

And, if the FF and m43 sensors have equivalent resolution, in MP, then f11 on m43 will suffer the same level of diffraction as f22 on the FF camera.

Your statement that m43 will suffer more from diffraction than FF really isn't accurate.

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Sorry, but for green light (550 nm) the size of the Airy Disk is

f Stop = Airy Disk
1.2 = 1.6
1.4 = 1.9
1.8 = 2.4
2 = 2.7
2.8 = 3.7
4 = 5.3
5.6 = 7.5
8 = 10.7
11 = 14.7
13 = 17.3
16 = 21.3
18 = 24
22 = 29.3

That relationship does not change because of the format of the camera or the focal length of the lens you're using. The only thing you can change that will make the values any different is the wavelength (color) of the light.

... And since you're at f/11 instead of f/22, you can take advantage of faster shutter speeds or lower ISO that can more than make up for the sensor difference.

In actual fact, there may be even more advantage to going to an even SMALLER sensor if you frequently "need" tons of DOF.

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No, the size of the airy disk doesn't depend on the format, but it's effect on sharpness does. Well, more accurately, the effect depends on the pixel pitch of the sensor, and smaller sensors generally have much finer pixel pitch than do larger sensors. Not always, of course; a high resolution FF sensor, like that in the D800, probably has a finer pitch than the 12 MP m43 sensors. But if you assume similar MP counts, a larger sensor will have larger pixels, and a greater pixel pitch, than a smaller sensor.

That means that an Airy disk of a given size will cover more pixels on the smaller sensor, and have a greater effect on image sharpness.

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Exactly, so the OM-D can really only take advantage of it's higher resolution sensor relative to the earlier m43 cameras at relatively open (f 5.6 or less) apertures. After that the size of the Airy Disk will limit them all to about the same amount of resolving power. It might have more pixels but there's not any more information coming from the lens than what the 12MP cameras can capture.

No, no, no. Assuming similar sensor resolution, diffraction on the OMD will be exactly the same as a FF camera at the same DOF. Exactly. The. Same.

To repeat: 35m at f11 on an OMD will have the same DOF, and the same level of diffraction, as 70mm at f22 on FF.

You can keep trying to claim that m43 is inferior, but the facts don't support the conclusion you're trying to reach with them.

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Diffraction Limited Photography: Pixel Size, Aperture and Airy Disks

According to their advanced calculator on a 16 Mp 4/3 sized sensor anything f8 and over is diffraction limited if viewing a 10" print with 20/20 vision. The answer depends very much on how one sets the question.

For 12 Mp they give f11 as the answer.

And if f/22 on a FF camera doesn't give the DoF you need—then what? I still don't see any "huge advantages" of mft over 35mm when large DoF is desired.
In practice with current cameras and lenses it is accurate. The smallest aperture most lenses have is f/22 for both mft and 35mm lenses, and at f/22 mft will suffer from diffraction more than FF.

I think the advantage comes in handheld situations. If you are shooting a scene with the same field of view that requires 1/FL F8 on both a FF and m43 then the m4/3 will have the deeper DOF advantage.
Example:
-FF 50mm @ F5.6 focused at 15ft will have 9.6ft DOF (3.4ft before 6.2ft behind)
-m43 25mm @ f5.6 focused at 15ft will be past hyperfocal so everything from 7ft on will be in focus.
(got these numbers from a phone app, please advise if any errors)

Now the farther off you are focusing it will become less of a issue, or if you are shooting with a tripod, but under some circumstances there is a real advantage to m43 in terms of a deeper field. Just like under some circumstances there is a real advantage to the shallower depth of field offered by FF.

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