I'm not sure if I've read that one, although it does sound vaguely familiar: based on the description that particular magazine would have been a few years too early for me.
I see that it's been asked about here too: https://scifi.stackexchange.com/---/please-help-me-id-this-70s-sf-horror-story-about-a-monster-in-a-little-box
There were definitely some pretty memorable stories in those magazines. I'm pretty sure I read "The Man Who Sold Rope to the Gnoles" in one of them. ( https://d-infinity.net/---/man-who-sold-rope-gnoles
Short stories are often tricky to pin down, though, especially if the title doesn't stick in your head (like the Gnoles above). I couldn't remember what it was called, just the bit about everyone breathing together, so it took me years to stumble across Orson Scott Card's "Deep Breathing Exercises" ( http://www.williamflew.com/omni10d.html
) in one of his anthologies after originally reading it in Omni. (Found it again tonight through the scifi.stackexchange site above.)
The other story I can never remember the title/author/source of is about a man who dreams about a mysterious pillar coming out of the sky and causing mass destruction. Eventually he learns that the dream is a message from tiny creatures on the back of his hand - every time he scratches between two of his knuckles he's cutting huge swaths through the landscape. In the end he decides not to scratch/wash his hand for something like a week - the critters are on a different time scale so he figures that will give them a few thousand years to figure out a solution.