Samsung Display announced it launched a new project (called EnDK) to develop the world's first 11K mobile display, that will achieve 2250 PPI(!). The Korean government will support this five-year project with a $26.5 million grant. Samsung is collaborating with 13 companies on this project, and they expect to show the first prototype by the 2018 Pyeongchang Winter Olympics.
This seems crazy at first for a mobile device - after all the human eye cannot physically distinguish a higher than 2K resolution in a mobile phone. But Samsung explains that this "super dimension" display will enable an "optical illusion" that will enable 3D images.
It's not actually clear if the display will be an OLED one, but it's highly likely. Samsung's current highest-density OLED in production is the 5.1" QHD (2560x1440, 577 PPI) Super AMOLED display used in the GS6. Samsung is also rumored to be developing a 5.9" UHD display for the upcoming Galaxy Note 5 - and if true it will achieve a 746 PPI.
The world's highest density display is Sharp's 5.5" UHD (4K) IGZO LCD display (2160x3840) - that achieves 806 PPI. In November 2014, Japan's SEL demonstrated a 2.8" 2560x1440 panel prototype - that reaches a PPI of 1058. This prototype panel is based on SEL's CAAS-OS (C-Axis Aligned Crystalline Oxide Semiconductor) backplane, and a WRGB structure.