OnePlus managed to integrate a fast charging solution into the OnePlus 3, but it's a non-standard one called Dash Charge. It's a licensed version of Oppo's VOOC, so it requires special code on the device and hardware in the adapter. Some OP fans were bummed that custom ROMs would not be able to support Dash Charge, but that will change next month when OP releases the code.
Right now, any ROM built for the OnePlus 3 would only be able to charge at standard speed, even if you plug the phone into a Dash Charge cable. That's because the code open sourced by OP does not contain the proprietary VOOC features. A OnePlus staffer has commented on GitHub to say the necessary code will be released by the end of July, but it won't be completely open source.
Some parts of VOOC/Dash will be kernel source, but the stuff everyone cares about will just be a binary. ROM makers should be able to use the unmodified binary to enable Dash Charge in their creations. So, the tech won't be completely open source for people to tinker with, but that's better than not having Dash Charge in ROMs at all. Note, this won't enable anyone to make accessories as the hardware is still proprietary. This is just about software support.