LG just recently passed Apple's quality test, so atleast this first batch of iPhones Xs and Xs Max should have the Samsung's OLEDs.
Later iPhones and replacement screens will probably be LG too.
LG's OLED screen on its own V30 and the Google Pixel 2 XL have received mediocre reviews. Here's a photo from an article in Ars Technica from a year ago that compared the LG V30 with a Samsung Galaxy S8. Of course, it's always possible that LG has improved its quality in order to qualify as an Apple supplier.