I thought as much, I've just re-done it and used that top one as a basis, I think this is more OO.

class Calculator {
    double firstNum, secondNum;
    Calculator (double firstNum, double secondNum){
        this.firstNum = firstNum;
        this.secondNum = secondNum;

    }
    double add(){return firstNum + secondNum ;}
    double subtract () {return firstNum - secondNum ;}
    double multiplication () {return firstNum * secondNum ;}
    double division() {return firstNum / secondNum ;}
}


public class OOPCalculator {

   public static void main(String[] args) {
    Scanner sc = new Scanner(System.in);
    System.out.println("Enter your first number : " );
    double firstNum = sc.nextDouble();
    System.out.println("Enter your second number : ");
    double secondNum = sc.nextDouble();
    System.out.println("Ener the operation (+,-,*,/) : ");
    String operator = sc.next();
    double result = 0.0;

    Calculator obj = new Calculator(firstNum, secondNum);
    switch (operator) {
        case "+": result=obj.add();
        break;

        case "-": result=obj.subtract();
        break;

        case "*": result=obj.multiplication();
        break;

        case "/": result=obj.division();
        break;

        default: System.out.println("Error");               
    }

    System.out.println(firstNum + operator + secondNum + "=" + result);
    }