Help me in solving PYTH99A problem

My issue

answer here

My code

def count_os(input_string):
    o_count = 0
    for char in input_string:
        if char == 'o':
            o_count += 1
    print(o_count)

# Example usage:
count_os("balloon")

Learning course: Python with Data structures
Problem Link: CodeChef: Practical coding for everyone