Google's VR vice president, Clay Bavor, revealed that Google has partnered with "one of the leading OLED makers" on a secret project to develop a high-end VR display. The OLED display will support a resolution of 20 million pixels per eye.
This is a very high resolution - a 4K display has 8.3 million pixels, so it means 2.5X the number of pixels in a 4K display. It terms of display density (which depends on the panel size of course, which we assume to be around 3.5"), this could be around 1,800 PPI.
A PPI of 1,800 is a lot - but in May 2016 Samsung already demonstrated a 5.5" 3840x2160 - which reaches 806 PPI. According to reports Samsung were able to produce a 1,200 PPI AMOLED in March 2017 - and the company's goal is to reach 1,500 PPI.
In November 2016 Sunic Systems announced that it developed an plane-source evaporation-FMM based AMOLED production process that can reach very high densities. The 100um mask announced in November can achieve a PPI of 1,500. This may be the system that Samsung is now testing. Sunic says that eventually this technology will enable even 2,250 PPI. This will certainly be enough to support Google's 20Mp target.
So a 1,500 PPI is certainly possible. A "leading OLED maker" is likely to be either Samsung Display or LG Display. Last month it was reported that Google wants to invest $872 million in LG Display in order to secure flexible OLED capacity from the Korean OLED maker.
Google also announced that it is developing a VR LCD display in collaboration with Sharp, with an aim to create an LCD with a high response time and high resolution. This will be interesting to watch - currently all high-end VD headsets adopt OLEDs, mostly due to the high response time.