I don't want to take anything away from the image, it's great! And I could certainly be wrong.
There certainly is OIII in M17 and M16. It's just that the haze in the background area is much too strong, IMHO, to be OIII. In your excellent Lagoon image the O2 is clearly present around the nebula, but not really in the background. The M16-M17 image OTOH has that haze extending across nearly the whole image in a rather linear way. This DSLR image of the same area from the dark skies of Namibia does show some O2 in the background, but it is not nearly as strong or as linear as in your image.
I probably shouldn't have said anything, because it really is irrelevant. If you are happy with the image, and you should be, then my comment was pointless. 