Apple started adopting OLEDs in the 2017 iPhone X, and it is expected that it will introduce one or two new OLED modules in its iPhone lineup in 2018 (which will likely include a total of 3 phone models).
A new report from Korea's ETNews suggests that Apple decided to exclusively use OLED displays for all of its iPhone models in 2019. As OLEDs provide a better image quality, and are also thinner, lighter, flexible and more efficient than LCDs, it is only a matter of time until all of Apple's iPhones and mobile device make the switch to OLEDs.
Of course a major drawback of OLED displays to Apple is the higher price, especially as Apple still depends on Samsung as an exclusive supplier. By 2019 this could change, of course, and Apple might be able to secure OLEDs from LGD, and perhaps from BOE or other makers in China as well.