When building mobile apps for Android, take care to checkout the current stats on the number of users using the different versions of Android OS. As on date, Oreo is used by less than 0.7% of Android Users as on 8th January 2018. To reach out to the optimum group of Android users, it is advisable to look at building your application for KitKat all the way till Nougat, i.e. KitKat, Lollipop, Marshmallow, Nougat.
data from android website as on 8 Jan 2018
For iOS on the other hand, there are 2 versions that you have to cater to. iOS 10 and iOS 11. Here are the stats for the same
data from apple developer website as on Dec 2017