Just a simple ternary using the two loops :

           for(int i = 0; i < 10 ; ++i){
                for(int j = 0; j < 10 ; ++j){
                    System.out.print(j <= i ? 'x' : 'o');
                }
                System.out.println();
            }

It's a simple matrix that you want to split by a diagonal. The logic is simple. If x (aka i) > y (aka j), you are one the right of the diagonal.