I used to buy
Omni at newsstands as a young dude in its heyday, and I remember "Sandkings", IIRC my 1st exposure to GRRM.
Unfortunately, don't recall the particular story EverSin_SF is seeking.
Omni was put out by the publishers of
Penthouse magazine, probably for those too young to buy skin mags, and those who
really did want to read the articles, without the associated 'baggage'. It's probably worth checking for this kind of inquiry during its run.
Added:
Here's a link to an archive of Omni, which I believe is legit.