def octal_to_binary(octal_number):
    """
    Convert an octal number to binary.

    Args:
        octal_number (str): The octal number as a string.

    Returns:
        str: The binary representation of the octal number.
    """
    binary_number = ""
    octal_digits = "01234567"

    for digit in octal_number:
        if digit not in octal_digits:
            raise ValueError("Invalid octal digit")
        
        binary_digit = ""
        value = int(digit)
        for _ in range(3):
            binary_digit = str(value % 2) + binary_digit
            value //= 2
        binary_number += binary_digit

    return binary_number

if __name__ == "__main__":
    import doctest
    doctest.testmod()