Update (Nov 2, 2018): Blizzard confirmed that Lucio-Ohs are entirely real at BlizzCon 2018. The cereal, which is being made in partnership with Kellogg's, will be available in the United States this December and will come with in-game loot.
Original story (October 8, 2018): A listing at a food service information website has leaked that Kellogg’s is most likely making real one of Overwatch’s best in-game jokes: Lucio-Ohs cereal, based on everyone’s favorite hover-roller-blading DJ. First noticed by the folks over at The Junk Food Aisle, it seems like the cereal is not only real, it’s coming December 3rd, and you can earn a Loot Boost from each box. (A loot boost, for those unfamiliar, gives you a bonus loot box when you next level up.)
We’ll keep an eye on it in case it turns up in stores, though the tan-and-green rings in sonic vanilla seem more than likely—given Kellogg’s recent novelty cereal promotions include Rick and Morty, Deadpool, and Reptar. If it's anything like those, however, it'll only be released in the US.
Aside from The Junk Food Aisle’s post and the food services listing, however, there’s little evidence at this point that it’s entirely real. The leak seems quite plausible, as Blizzard is currently working with Kellogg's other brands on an identical loot box promotion. Activision Blizzard also has a history of running similar promotions through Taco Bell, Rockstar Energy, and other brands for games like Destiny 2.
...They really should be called Luci-O's though, shouldn't they?