LG, however, has reportedly hit production hiccups, and Apple has taken the rare step to make it come up with a third prototype of the alleged 6.5-incher that leaked not long ago. LG doesn't yet have the amount of modern production equipment Samsung has installed in expectation of huge OLED iPhone orders, and apparently can't meet Apple's stringent quality and yield targets.
The new iPhones should be ramping up production in July at the latest, so that they can be announced in September, and drop a week or two later. Thus, if Apple doesn't solve LG's factory conundrum, it may be forced to rely on Samsung for the bulk of its OLED display needs yet again. Apple is forecast to make 100 million of the 2018 iPhone crop, and half of those would be OLED models, so Samsung might soon have something to fill the idle capacity that was left when Apple cut the iPhone X display orders due to lower than expected sales for the quarter.
source: WSJ (paywall)