First OLED iPad may not arrive until 2022

Per the report, the transition will begin with a few models. The orders will apparently go to Samsung and LG, who also make OLED panels for the iPhone 12 series.
According to an earlier rumor, Apple has demanded higher specifications for the OLED panel meant for the iPad Pro when compared to the screens supplied for iPhones. Today's report provides more clarity on that.
Apple is expected to use hybrid OLED units which are thinner than conventional panels and are made using a combination of flexible and rigid OLED technologies. Samsung and LG have branded these panels as UT (Ultra Thin) and ATO (Advanced Thin OLED), respectively.
Samsung Display will reportedly start supplying UT panels for the iPad in 2022, and the chaebol has already started working on facilities and equipment that would be required. LG Display will commence shipments in 2023. The report adds that full scale manufacturing will also begin in 2023.
mini LED iPads will likely precede the arrival of OLED ones

OLED equipped Samsung Galaxy Tab S7 Plus
As you may already know, all iPads in Apple's current lineup sport LCD panels. The latest report corroborates earlier rumors that said some 2021 iPad models will use mini LED, presumably a 12.9-inch iPad Pro, a 10.2-inch iPad, and a 7.9-inch iPad mini. To be more specific, up to 40 percent of iPad models coming next year could feature miniLED. The tech is expected to offer similar benefits to OLED but without downsides such as burn-in.
That said, competitors like Samsung and Lenovo already make OLED tablets, so it wouldn't be surprising if Apple makes the switch too.