In the last few years we witnessed an interesting evolution of smartphone display sizes: from bezel on the top and the bottom, to notch-type displays, to full-screen displays with pop-up selfie cameras and also displays with round laser-cut holes.
OLED makers have successfully integrated fingerprint sensors behind the OLED panels, but integrating a camera completely behind a transparent part of the OLED display is more difficult. Earlier this year Samsung Display estimated that it will take at least 3 years for this technology to reach the market - but yesterday both Xiaomi and Oppo unveiled smartphone prototypes with selfie cameras under the OLED display.
Both companies did not unveil any technical details or any more information regarding the readiness of this technology. It could be that this is years away, as SDC estimates - or it could be that Chinese OLED makers are more advanced than Samsung in this area. Time will tell!