Question about const member functions in c++

Can anyone explain me this error? Here is the code:

class O{
    unsigned x;
    unsigned y;
    public:
        O(unsigned x_ ,unsigned  y_): x(x_), y(y_){
        };
        O& operator+= ( O & object){
            x += object.x;
        }
};

class A {
    O item;
    public:
        A(O i): item(i){;
        }
        void fun() const{
            O j(2,3);
            j += item;
        }
};

int main(){
    return 0;
}

When I try to compile I get this error:

In member function 'void A::fun() const':
[Error] no match for 'operator+=' (operand types are 'O' and 'const O')
[Note] candidate is:
[Note] O& O::operator+=(O&)
[Note] no known conversion for argument 1 from 'const O' to 'O&'

Can anyone explain me this? If I add const qualifier to the argument of the += operator, it compiles. So I think the problem is that I'm passing a reference to item to the += opertor, which is non const, inside the const function fun(). Can anyone explain me why this is illegal, and how I can avoid to do this kind of mistakes, if for instance there is some rule of thumb to follow when using const qualifiers, etc..?

This member function

  void fun() const{
        O j(2,3);
        j += item;
    }

is a constant member function. So members of the object to which the function is called are considered as constant members in particular the data member item in this case is considered as it was declared like

const O item;

In this expression

j += item;

there is used the member function

    O& operator+= ( O & object){
        x += object.x;
    }

that accepts a non-constant reference to an object of the type O. So in fact you are trying to bind a non-constant reference to a constant object. So the compiler issues an error.

The parameter of the above operator should be declared with the qualifier const and has a return statement as for example

    O& operator+= ( const O & object) {
        x += object.x;
        return *this;
    }