It's because OPPO/OnePlus use proprietary charging protocol & chargers (OPPO SuperVOOC / OnePlus Warp). It's not compatible with PD which most laptops use nowadays.
I found the the 8t can warp charge with any usb c cable with the included charging brick. But slow charges all other devices with the included brick. I use rhinoshileds usb c cable and have also tried apples usb c to c cable and both w65 charge my 8t.