OnePlus will likely add OLED DC Dimming feature to an upcoming Oxygen OS beta release as a completely optional feature within the OnePlus Laboratory or within the Developer Options sub-menu.
Due to the technique used by OLED panels to control backlight brightness, display flicker tends to be more pronounced at the lower brightness levels. In basic terms, at 100% brightness flicker is less obvious as the panel emits light almost continuously. At low levels, the display emits light in waves.
This can potentially cause eye-strain for those that are particularly sensitive to light. Naturally, this might not be an issue for those who use ambient brightness settings, but it still poses a problem for many users of OLED displays.
The DC Dimming technique essentially adjusts the DC current to dim the display. The only problem with this method is that visible display quality can suffer, which is arguably why the feature is not found on more than a handful of devices at present.