Some users are having Bluetooth audio issues on Android Oreo according to a growing collection of comments from users over on the Nexus Help Forums and the Pixel User Community. At first reports were coming from users simply mentioning the problem, but it wasn't enough information for Google to go off of so it began requesting users offer up more information, asking for the year, make and model of vehicles, the brand of headphones people were using, and the brand of Bluetooth speakers people were using. In addition to those things Google has also been requesting that users mention what the issues or symptoms were with each category so it can begin to try and narrow down what the problem might be.
This appears to be happening on both Nexus and Pixel devices given that the complaints have spread across both forums, though when it comes to vehicles, speakers, and headphones it appears more people are having problems with Bluetooth audio on Android Auto more than anything else. It also looks like the issues are spread across a wide range of different vehicle brands and models from various years, so it's not likely that it has much to do with the incompatibility of certain vehicles.
There seems to be a handful of issues with the different categories Google was requesting information about, with people complaining that media content isn't displaying as it should on Android Auto or that the audio is showing that it's paired but it isn't playing, while people using Bluetooth with speakers and headphones have been complaining about the audio cutting out every 15-20 seconds. Some users aren't able to use Bluetooth at all, with complaints surfacing that turning it on just results in it turning back off a second or two later. Google began requesting more information from users yesterday so it's just started to gather that information, which could mean it will still be a little while before any resolution is given or before a fix is pushed out. If that's the case, then unfortunately users will be without Bluetooth audio until then, or may need to revert back to Nougat for the time being if they really need the audio to work.