Android 8.0 Oreo is already hitting Developer Preview devices. Google officially unveiled Android 8.0 today and did mention that the software would be hitting AOSP today as well, while mentioning that Pixel and Nexus devices are in carrier testing and should be rolling out soon to those devices. As for those who were included in the beta and already on the beta software, the software push has already started, with users of various device types on the beta receiving the update alert.
The image below is a screenshot from an unlocked Nexus 5X, with the download being nearly 36MB in file size, though worth keeping in mind is that the file size is likely to be a fair amount larger for those who aren't already on Android O. If you're on Nougat of any version, chances are the download file will be much bigger, which would mean it will take longer to download. Considering this, you'll definitely want to have at least half battery life, and make sure you're connected to Wi-Fi if you don't want to eat up your 4G LTE data, that is if you have any limits in the first place.
In addition to this particular build of the software rolling out to users who were on the beta software already, Google has already posted the factory and OTA images for download up at its official web page, meaning anyone who isn't on the beta and received the update download, it's still possible to flash the factory images or OTA files and install the new software anyway, if you have one of the supported devices. The build you should be seeing will be OPR6, ad if you aren't seeing your own device getting the update alert even with the Android O beta already being on your device, you could always try checking manually if it's available to download, and a restart of the device couldn't hurt either. As for what's new, Picture-In-Picture mode, Smart Text Selection, better security, and Notification dots are just a few of the things to look forward to once the software is installed on your own phone, though these definitely won't be the only changes to keep an eye out for.