This behavior is inherited from BaseException:

https://docs.python.org/3/library/exceptions.html#BaseException

    >>> str(BaseException())
    ''
    >>> str(BaseException('spam'))
    'spam'
    >>> str(BaseException('spam', 'eggs'))
    "('spam', 'eggs')"

Python 2 OSError special cases 2-3 arguments:

    >>> str(OSError(*'123'))
    "[Errno 1] 2: '3'"

    >>> str(OSError(*'1234'))
    "('1', '2', '3', '4')"

Python 3 OSError special cases 2-5 arguments (adding winerror and filename2):

    >>> str(OSError(*'12345'))
    "[Errno 1] 2: '3' -> '5'"

    >>> str(OSError(*'123456'))
    "('1', '2', '3', '4', '5', '6')"

The base behavior is implemented by BaseException_str in Objects/exceptions.c:

    static PyObject *
    BaseException_str(PyBaseExceptionObject *self)
    {
        switch (PyTuple_GET_SIZE(self->args)) {
        case 0:
            return PyUnicode_FromString("");
        case 1:
            return PyObject_Str(PyTuple_GET_ITEM(self->args, 0));
        default:
            return PyObject_Str(self->args);
        }
    }