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Core rules state Open order (and Closed Order) needs to have 2 or more models.

But it is possible to take a unit of 1 models in many cases, such as Chaos spawn.

So the question is what formation are they in if you have a unit of 1?
or
if a unit is reduced to 1 model after combat?

There are no explicit rules to resolve this situation but the only way to make it work in-game is to ignore the parts of Close and Open Order that says "2 or more models".

That or treat all single models as Skirmishers which causes weirdness with Light Chariots.

Otherwise, no RAW answer until they errata. IF They errata.

I remember the good old days when chaos spawn used to be much more fearsome:

https://blogger.googleusercontent.com/img/b/R29vZ2xl/AVvXsEg-roQCy0wR90LwTJkSQFFXqaG67tk3Pk0WbQEhFE-wFV7LYrpugVTHNpMrSdxtUH3ZM0hrf9K0p2AHLa2Xs0rbS0aXkB7fSEXyS5YrziU61RW07U-_GZ7TB6GMOsgviCrya03jK1SlCqDX/s1600/GratuitousColour.jpg

It remains open order which is to designate how it interacts with terrain and lets you know that it still sacrifices movement to perform maneuvers even with random movement rule. And has quick turn.

I'll add random movement is pretty busted but thats a different conversation.

Serious answer - Send the FAQ team an email clearly stating your issue. The more people ask the more likely an answer.

oldworldFAQ@gwplc.com

How i read and play it, you get no combat bonus from open or closed order but you have to use al other rules for the named formations.

For what it's worth (admittedly not much), GW's battle report showed them adding the bonus for Closed Order to single-model units.