No you will not get OK response in once AT press.

If you have an external modem Try it your self Open hyperterminal -> Create a connection direct to com port by selecting any baud rate you wish less than or equal to 115200. no type AT and press enter do it just onece and you will get OK. And I don't who the hell has given me that -1 on a correct answer. I am sending a mail stating the same let's see if he can give me the name of that person.

Regards,
Prahlad Purohit