Visionox developed a new RGB pixel arrangement for OLED technologies that allowed them to reach 570 PPI. The company refers to this new technology as Z-Type arrangement and it includes 3 sub-pixels (RGB) per pixel (unlike Samsung's Pentile displays). The sub-pixels are densely packed (the aperture ratio seems very high) in a way that does not suffer from jagged edges.
Visionox told me that they cannot yet say when such displays can be commercialized, as there are still manufacturing challenges to overcome. In the meanwhile the company applied for both Chinese and international patents.
In any case, 570 PPI is very impressive. The AMOLED with the largest PPI in production is Samsung's 4.99" Full-HD panel (used in the GS4) that features 441 PPI. Earlier this month AUO announced that they developed a 5.7" WQHD (2560x1440) AMOLED - that's a PPI of 513, but that display is not commercial yet.
Sharp, and SEL developed a 13.3" 8K CAAC-OS OLED that achieves 664 PPI - but this one hasn't been unveiled yet and Samsung Display announced a while back that they are developing a 5.2" WQHD panel which will have a pixel density of 560 PPI and are also planning a UHD panel that will have a PPI of 860 (!).