Tokolozi , Oct 22, 2020 : What I remember is that it might be in a future update. Since currrently it's only in hydrogenOS. I don't have the device so can't say for sure.
kbszh123 , via OnePlus 8T , Oct 22, 2020 : Thanks man! It's there for 8 but not for T. Waiting for update
V1595960753315 , via OnePlus 7T Pro , Oct 22, 2020 : Go to Setting --> Utilities-->OnePlus Laboratory You will see the open of DC dimming
V1604239098234 , Nov 1, 2020 : Since the DC dimming feature is available in HydrogenOS, I believe it will be added to OxygenOS soon. Meanwhile I have found a way to turn it on in OxygenOS by running this command in ADB shell: am start-activity -a oneplus.intent.action.ONEPLUS_LAB_FEATURE_DETAILS -e oneplus_lab_feature_key oneplus_dc_dimming_value Use it at your own risk!