Suppose we are using int main() and after that in the last of the program why we are writing that return o;

Answer 1

Since main() has return type of int, it has to return a number. return 0; indicates safe or successful termination, while anything else means abnormal termination.
If you don't add a return statement to main(), the compiler will insert a return 0; for you. But it's a common convention to write it explicitly.
You can always make main() void instead, though it's also a convention to write return; at the end anyway.

Answer 2

in int main () int is the return type of main function. it must return something . but if we have nothing to return we should simply return noth8ng which is return 0 .. i.e. need to write return 0