APK installation on Android O now comes with a progress bar so you can see how much time you have left before the APK file is finished installing on your device. While this might be a rather minor change, it's one that is likely to be a nice little perk for those who frequently install apps from an APK file (an outside source) as opposed to the Play Store. That being said, since this is the first developer preview for Android O Google could always remove this change and choose to leave things as they are in Android Nougat and below.
Even though the progress bar is now present, it's just a progress bar that shows you from beginning to end where you're at with the installation process of an APK file. When you install an app from the Play Store, if you sit on the app's listing page and watch the install, you not only get the progress bar to look at but you also have a percentage telling you how much of the installation has already finished. That isn't the case here with Android O APK installations. Of course, this again brings us back to the point of being the first developer preview, as Google could choose to add the percentage along with the progress bar in one of the later previews, so if you're hoping for that, don't give up hope just yet as there are three more previews to go through and then the final release that is due for launch in the third quarter of the year.
In addition to the progress bar Google has also added in the ability to cancel an APK installation with a dedicated button that sits in the bottom right corner of the screen. This is also something you can find when installing apps from the Play Store and you will no longer have to wait for the APK to finish installing only to uninstall if you changed your mind before it completed. Simply tap the cancel button and go about your day. If you're already using the Android O developer preview on your compatible device, then you should be able to see this in action first hand.