Ever since users started making positive reviews about dark mode, especially on devices that have monitors AMOLED, more and more applications are starting to use it. However, many applications have not yet decided to join the "dark side".
A settings page for forced dark mode has also been detected. But still the applications supported by mode are not confirmed.
This discovery leads one to conclude that OnePlus is trying to bring a forced dark mode to OxygenOS. Which means that users should use applications like Facebook or the Snapchat with a dark theme, even if this option is not available from the applications themselves.
Recently and h WhatsApp offered the ability to run dark for applications Android and iOS. To enable dark mode on WhatsApp, you can refer to this guide. So there is a high chance that future OxygenOS updates will provide the forced dark mode for more applications.