The first developer preview of Android O was only out for a couple of days before Chainfire created a new root method for it. For the time being, only the Nexus 5X and Nexus 6P are supported by this first SuperSU ZIP, as the Pixel's dual-partition layout has created some hurdles, though Chainfire is surely working on overcoming those at some point soon.
This root method is still in its earliest stages of development, so you should expect a few bugs here and there. Despite the lack of major visual changes in Android O, behind-the-scenes tweaks have slightly changed the way rooting will work. SELinux rules have been tightened some more, and kernel changes have posed some problems. But, if you're a tinkerer and you're running Android O as your daily driver, then rooting your phone is a must, so we'll cover the process below.
First up, you'll need to install TWRP. To do that, start by grabbing the newest IMG file from one of the following links, but make sure to download the right one for your device.
From there, provided you've already unlocked your bootloader, just put your phone into bootloader mode, then plug it into your computer with a USB cable. Make sure you have ADB and Fastboot installed, then transfer the twrp.img file to the platform-tools folder inside of your ADB installation directory. From there, open a command prompt in the same directory, then type fastboot flash recovery twrp.img and hit enter. When you're done, go ahead and reboot your phone.
If you need more help flashing the TWRP image with Fastboot, we've covered the process in detail at the following link. While the guide mainly deals with flashing factory images provided by the manufacturer, the same principles apply to flashing the TWRP image file.
Now you'll need to download SuperSU on your Nexus 5X or 6P. There are currently two versions of SuperSU: The 2.79 SR4 version, which is a beta release for SuperSU, and the 2.79 SR4-ExtraDelay version, which delays the SuperSU daemon on boot by 15 seconds.
The ExtraDelay version is there in case you have any problems with the SR4 version, since there is a known issue which causes a kernel panic on the tested Nexus phones. Go ahead and download both of the ZIPs onto your phone, just to be safe.
Next, reboot to TWRP by powering your phone off, then pressing and holding the volume down and power keys for five seconds. From there, press the volume down button twice to highlight the "Recovery Mode" option, then tap the power button to select it.
When TWRP boots up, tap "Install," then navigate to the Download folder and select the SuperSU ZIP without the extra delay. Then, swipe to confirm the installation, and when that's finished, tap on Reboot System and wait for Android to boot up (it might take a couple of minutes for the first boot with root).
After your phone has turned back on, make sure SuperSU is in the app drawer. If you're unable to get past the lock screen, that means you might have caught an Android O bug. To fix it, simply reboot to TWRP and flash the version of SuperSU with ExtraDelay at the end of the file name, then you should be all set to go.
Be sure to let us know about your experience rooting Android O, and remember that some root apps might be buggy until the developers are able to give them an update.