Google rolled out the developer preview of Android v8.1 yesterday. And while factory images were live since yesterday, the OTAs are just rolling out today. The manual OTAs that were available since yesterday have been taken off due to some flashing problems.
Hence, the only option to hop on to v8.1 is through the Beta program. Or you could flash the factory images, but that will require you to unlock the bootloader. Alternatively, you could just wait for the OTA to drop in.
Google is right now directing OTA page visitors to the factory images page. There were some flashing issues with the manual OTAs, however using the factory images is fine.
Here’s what a Google rep said on the manual OTA issue:
So, if you’re on a bootloader-locked device, right now your only options for moving to the 8.1 developer previews are to register for the beta program or wait for Google to bring back the OTA files for sideloading. At least now the Beta Program OTA appears to be rolling out, so you shouldn’t have to wait too long if that’s the route you take. In addition to the reports we’ve received, we can confirm that an OTA from the Beta Program registration has hit one of our devices.