Android O is introducing separate accessibility and media volume controls to make it easier for users to adjust up or down either volume to the setting that they wish. Prior to Android O, adjusting the media volume up or down using the quick access sliders meant that you had to adjust the volume for accessibility to the same level, but that's changing which is sure to be hugely beneficial to anyone that has a need for accessibility volume to be at an appropriate level so they can hear what it interacts with, even if at the same time they need the media volume to be at a lower, more respectable level, say on public transit.
Once Android O is installed on compatible devices, or for anyone who is already running the latest developer preview version of Android O on a compatible device, after you hit the volume up or down button and then tap the drop down arrow that reveals the additional volume slider bars, you should see a new one dedicated completely to Accessibility sitting just below the slider bar for media so that you can manage it as you need to.
While this is definitely going to be a useful change for a number of users, it's not the only adjustment that Google has made in Android O that is geared towards the accessibility functions of the operating system. In Android O Google has made it easier to get to and enable the Accessibility shortcut. All you have to do is press and hold on both the volume up and volume down buttons at the same time for a total of three seconds, and this should enable the shortcut for you. Disabling it happens in just the same way so there's no confusion between the two. The reason Google is making this possible is so that the accessibility shortcut can be activated easily and quickly no matter what users are doing on the phone. With this method they can be on any screen and get to the shortcut if they need to without having to leave the screen they're on. All that said, you still have to toggle the accessibility shortcut on from the settings menu so you can activate when you need it, so it's important to remember that step. Other recent changes to the accessibility-focused functions in Android O include the optimized select to speak function for Talkback that Google implemented as well.