Answers: First - why do you ask? That might aid reader to answer more completely.
In common, most operating systems contain a kernel (which includes functions close to scheduling, post dispatch, memory supervision, and much, much more).
Depending on the operating system involved, (and who's chitchat give or take a few it) the second primary component might be an I/O carton, but tons techies (me included) really consider that more a subdivision of the hardware (sometimes referred to as the BIOS or Basic Input Output System) since it's contained in the PC's hardware. Others might consider the File System to be a second piece of the OS since tons operating systems consent to you select which one to use.
Others will transmit you that in that are more than two, or even lone one primary component.
As I said, it adjectives depends on why you want to know or how you intend to apply the information. How granular (or detailed) do you want to be?
Regards -
The two principal internal components of an OS are:
1) kernel
2) user interface