Until now, OEMs have been free to use whatever Linux kernel they wanted to create their own version of Android, there were other tests to pass, but Google did not care what kernel number you used.
However Android devices running Oreo must use at least kernel 3.18, but there are more specific requirements to meet as well.
Google explains on the Android Source page: "Android O mandates a minimum kernel version and kernel configuration and checks them both in VTS as well as during an OTA. Android device kernels must enable the kernel .config support along with the option to read the kernel configuration at runtime through procfs."
It looks like Google has decided to do a little bit to fix some of its fragmentation problems, although this is a minor step.