Wang Xiang, a senior vice president at Xiaomi published this short video in his Twitter account, showing a new Xiaomi prototype that uses a foldable OLED. The devices folds in two places:
This is a very nice demonstration, although of course we have no way of knowing how close this technology is to commercialization.
Xiaomi says this is the world's first-ever double-folded phone (usually such OLEDs are referred to as tri-folded as they have three folded panels). This may be true, although we have seen such folded OLEDs in the past. Japan's Semiconductor Labs demonstrated touch-enabled tri-folded OLED prototypes as back as in 2014 (in collaboration with Nokia).
We do not know who provided Xiaomi with this specific OLED, but there are many display makers that are working or even already producing foldable OLEDs - including Samsung, Royole, BOE, Tianma, LG and others.