Every OS has its own set of APIs that allow an application to interact with its kernel. This enables an abstraction between the application and the API and also controls how much access an application programmer can add to his/her application. Take more information at Mova.io
Application software and operating system will not work at same time. Operating system supports the application software. Operating system does the jobs which are essential to work the application like loading,compiling etc.
When you use a program,, it sends commands to the computer. But the computer is just a heap of nuts and bolts and computer chips. It can't understand the commands by itself. So the operating system catches the commands and tells the computer what you want it to do.