It seems that after KitKat Google doesn't want to have another partnership with any confectionery maker, so it has decided to call Android O "Oatmeal Cookie" instead of Oreo. And while it's still speculation and Big G can surprise us with something else as well, the possibility of this name can't be denied as potential references to it have been found in Android source code.
Besides these mentions Oatmeal Cookie is also more likely to be the name because Google has historically used confectionery themed names for its Android versions. For example, Lollipop, KitKat, Marshmallow, Nougat – all revolve around the same theme. Oatmeal Cookie also fits this criteria. But so does Oreo, so who knows!
It was confirmed in latest preview of Android O that numerically it'll indeed be Android 8.0. However, the name was missing. So now we probably know what it may be. And while it's not as cool and popular as Oreo, things may change soon after it's announced officially by Google. Currently we're habitual to Oreo, but if it's called Oatmeal Cookie then eventually we'll get habitual to that as well.
But in the end it doesn't matter what Google calls it, because this version of Android is already equipped with more than enough solid features to make it successful. Some already popular ones include high quality Bluetooth audio, picture in picture mode, adaptive icons etc.
We'll come to know the official name in August, so there's a wait of one month only. In the meantime we can try getting habitual to this Oatmeal Cookie name as well!