The short explanation
An LCD has a backlight shining through a filter, so black is really very dim gray. Organic light-emitting diode panels have no backlight: each pixel emits light itself and a black pixel is off. Contrast is effectively infinite, and there is no halo around bright objects.
The trade-off is peak brightness. OLED televisions typically reach lower peaks than the best LCDs, which matters for HDR in a bright room.
Where you run into it
Because peak brightness is limited, tone mapping is doing real work on an OLED, and the luminance values advertised in the HDR metadata have a visible effect. Many of the LLDV settings people fiddle with exist precisely to get that mapping right on OLED panels.
Small OLED panels are also common as status displays on AV hardware, where their readability at a glance in a dark rack is the point.
How the DR HDMI 8K handles it
The DR HDMI 8K has a 1-inch 128×64 two-color OLED on its top. It cycles four info pages: the default page with FRL rate and RX/TX status, the RX EDID page, the TX EDID page and the unit info page with serial, firmware and IP. It is the fastest read on what a chain is doing without opening a browser, and it can be set to fade after a timeout (30 seconds by default), rotated 180 degrees or switched off entirely.
Want to test your own HDMI chain?
The DR HDMI 8K is sold through the HDFury.eu web shop.