Android O lets you add ringtones to the sound list so you can easily select third-party ringtones. While it's been possible to use third-party ringtones on your device since literally the beginning of Android, it's never been this easy. Google is changing that with Android O in their never ending quest to make as many things in Android as simple as possible. If you're running the first Developer Preview (and hopefully this carries through with future versions of O) you can go to select a new ringtone, and while you will still have just the ones that come standard with the software build, if you scroll all the way to the bottom there will be a new option to "Add ringtone."
This option will let you select different sounds from your files and after one is selected you can go back into the ringtone picker and it will then be in the list. This works for notification sounds and alarm sounds as well, so across the board any tone you have on your device can easily be modified. If you're not on Android O then your options are a little bit more limiting. While there isn't an option to add a ringtone or notification tone straight from the sound picker, you can install an app like Zedge which allows you to easily set new tones, or you can place the files manually in the folder where all the other tones are stored, though both of these options require a tiny bit of extra effort.
Now that this is a thing, and it probably should have been a thing long before now, developers who are making custom firmware will be able to implement this feature into their versions of Android software, and that should make those who like to flash custom firmware happy but there is no telling how long it might take before something like this is implemented into ROMs and such. It's important to remember that this is just the first Developer Preview, and as such it's entirely possible that this is a feature which could change or be reverted. However, there's probably no good reason why Google wouldn't keep it in place as it's pretty nifty.