OLED is an emissive display technology - which means that it emits its own light, in contrast to a reflective display - which uses an external light source - an ePaper display for example, or an LCD which is a display that blocks light from a backlight unit.
OLEDs are bright, and provide great image quality, and as of 2021, these displays perform very well under direct sunlight.
But this was not always the case. In early AMOLED displays, sunlight readability was very poor. The 2008 Nokia N85 for example, one of the first products with AMOLED displays, behaved very poorly in direct sunlight, as can be seen in the image below:
The problem in early AMOLED displays resulted from reflectance from metal electrodes in addition to relatively low brightness. Display makers however, soon upgraded their OLED displays to increase performance. Helped by new technologies and materials, OLED displays quickly overcame the sunlight readability problem. Today OLED displays actually perform better than the best LCDs in direct sunlight.