For fast charging, don't expect to find any standard USB Power Delivery or Qualcomm Quick Charge technology on the OnePlus 6. Rather the phone only uses its own brand of Dash Charge quick charging scheme over its USB 2.0 based Type-C connector. While the Dash Charge claims to support up to 20 watts output power for the OnePlus 6, let's see how it can be used to charge another device such as the USB Power Delivery based Apple 12.9-inch iPad Pro.
In most situation when there's just no access to AC power source, a dual role power enabled phone like the OnePlus 6 would come useful acting as a power bank to charge other compatible devices as well as being able to be charged itself. To demonstrate this, we connected a Apple 12.9-inch iPad Pro to the OnePlus 6 using a Apple USB-C to Lightning Cable to observe charging. Note the Apple iPhone and iPad Pro's that support USB Power Delivery can only consume and not supply power, therefore the OnePlus 6 needs to drain its own battery charge to support charging the iPad Pro. This requires the phone to have enough charge left in its battery for this operation.
Once plugged in with the phone's USB charging function turned on, the iPad Pro was observed to pull just about 2.4 watts from the OnePlus phone. While the iPad Pro is able to charge faster using its own Apple 12W based charger, for non USB Power Delivery charging, 2.4W can still be useful for a quick charge up when needed. Just don't expect charging to get any faster given the low power level as well as the iPad Pro's big battery. Click here to view more details on how the OnePlus 6 charged the Apple 12.9-inch iPad Pro.