Paranoid Android now has an unofficial build of the latest version for the OnePlus 5, courtesy of XDA Developers user hekefake, and despite being unofficial, the build seems to bring only a couple of minor bugs with it. All of the Paranoid Android features from the newest version that fans have come to know and love are present, including the new Pie, Color Engine, Pocket Lock, On The Spot Controls, and other favorites. Bonus features include the ability to pass SafetyNet out of the box, making it possible to play Pokemon GO and use Android Pay a lot easier, as well as recent task locking and screen burn-in protection, among other features. The only bugs reported at this time seem to be that the OnePlus 5's DCI-P3 color gamut option isn't showing up, and NFC payments aren't working right, though this is only the case for some users, with no real consistent factors having been found just yet.
The ROM is based on AOSPA version 7.2.2, which is in itself based on Android 7.1.2 (Nougat). There was no word from the ROM maintainer on whether this ROM will make the jump to Android O, but since the OnePlus 5 will likely get official Android O during its lifetime, that is quite possible. The OnePlus 5 also gets just a few device-specific features, which may end up in future versions. Those are sRGB support, alert slider customization, and offscreen gesture support.
Installing a custom ROM will require a custom recovery and an unlocked bootloader. Once you've checked off the "OEM Unlocking" option in your phone's Developer Settings, you should be able to flash TWRP recovery through Fastboot, or through an app like Flashify or the official TWRP app, if your device is rooted. Flashing a ROM, much like applying a device update, comes with some caveats, the main one being that you should never allow your device to power down during the flashing, or it could become inoperable. With this ROM and device in particularly, you may also want to avoid custom kernels. This ROM is made to work with the kernel it ships with, and flashing another one will lose you the support of the developers, and could break things like SafetyNet support and battery life optimizations.