Android O Developer Preview 4 re-enables the hide notification overlay option that was first introduced with the original Android Developer Preview build. If you've already updated to the fourth and final developer preview of Android O, which Google just recently pushed out to users today, then you'll be able to find this option in the notifications menu from settings, though it isn't directly available on the first screen as there are a few steps to go through before you find it. With it being a little more of an involved process to get to it, it's also easy to miss.
Once inside the notifications menu, simply tap the overflow menu button in the top right corner, then tap an App Info and then tap Show System. From here you can tap on Android System and this will bring up the toggle for disabling the overlay option which lets apps post notifications over other apps. It might not be available right from the front end of the notifications menu but it doesn't take more than a few seconds to get to right where you need to be, and if it bothers you that apps are able to post notifications over other apps then it's certainly worth taking the time to turn the feature off.
To some this might seem like a rather minor change for the latest version of Android O, but it will no doubt be an invaluable addition for some users. This of course is not the only change to come along with Android O's fourth developer preview, and it probably isn't going to be the last thing that changed between this version and the last one that gets discovered. Earlier today, a new Easter Egg was discovered with this latest version of the software, and there were a few small design changes that were brought to the lock screen. Alongside launching the fourth developer for Android O today, Google has stated that Android O is also officially coming soon, though it did not specify a date for the release of the stable version of the software that it aims to push out to the public a little bit later this year.