Earlier this month, OnePlus started rolling out OxygenOS 5.0 (aka Android 8.0 Oreo) to the OnePlus 3 and 3T. Meanwhile, Oreo for the newer OnePlus 5 is still in beta. If you have a 3 or 3T, and you'd rather not wait for the update to be pushed to your phone, you can now download the system images straight from OnePlus' website.
OnePlus has dedicated pages for both models with instructions for updating. It's a pretty straightforward process - download the .zip file, copy it to your phone, reboot your phone into the recovery, and select the zip file. After a little while, you'll be ready to enjoy Android Oreo.
The system images for both phones are around 1.5GB. You can download them at the links below.