What is the highest OG which is still fermentable?

I know that raw honey can ferment if it has enough moisture content and a warm environment.

There is an interesting thing that can happen resulting in snowflake yeast where yeast doesn't seperate during reproduction due to the bottle neck effect of the dense media it's in. This actually results in genetic mutations too. Pretty cool huh?

As far as an OG that will completely inhibit yeast. No idea, but it seems it would need to be cut with a knife.

For brewing the biggest OG recipes are with Wee Heavy (1.130), Barley Wine (1.120) and Iced Cider (1.180). None of which are "dry" styles, good attenuation is desired but just not possible to get dry because of the OG. Fermentation in these styles only reduce SG by 1.060-1.100 relative to the OG.

Yeasts can consume a lot of sugar in thier life, just not all at once. So if a yeast is tollerant to say 25% ABV, it will still crap out if thrown into a wort with that potential. Instead they need to be given additions of fermentables during fermentation to gradually get up to 25% ABV. This allows the yeast to strengthen its cell walls to deal with the increasing sugars and alcohols.

Interesting question. Lead me to some interesting Google results.

First is this paper regarding fermentation in the Tequila industry. Brief take home is they went to 170g/L of sugars, but found that supplemental nitrogen is needed for good yeast performance (i.e. yeast nutrient)

This second source sites distillers yeast going to 350g/L. I'd expect brewers yeast to be less hardy than that.

So it depends on your application I guess, but the yeast nutrient thing is important to consider.

Couldn't find a clear value for osmotic pressure imbalance. I'll keep looking.