Samsung has a virtual monopoly in the small-to-medium OLED display market which is why most major manufacturers who want to make the shift to OLED displays for their smartphones end up at Samsung's door. According to reports, Samsung may start supplying OLED displays to Xiaomi.
Xiaomi has reportedly signed an OLED supply deal with Samsung recently. It's apparently going to source OLED displays from Samsung for its next flagship smartphone that's due in 2018.
Samsung is reportedly going to supply Xiaomi with a 6.01-inch OLED panel with the first shipment starting from December 2017. Close to one million units of the OLED display will be shipped in the first month before an additional 2.2 million units are sent to Xiaomi in the following month.
Reports suggest that Xiaomi had initially planned to use a 5.49-inch flexible OLED panel from LG but the deal didn't go through as LG Display's new plant operation has been delayed.
Neither Samsung nor Xiaomi have confirmed this deal but that's to be expected, companies normally don't comment on rumors and reports concerning their supply chain.
Since Samsung supplies OLED displays to a wide variety of OEMs, it's certainly plausible that it might start supplying the panels to Xiaomi as well.