is 'Ok' important when sendin AT commands to modem
Yes, you can use a delay and ignore the response, but if it is a response to sending the actual SMS itself, then that could take some time.  You would have to send several SMS messages by hand to judge how long it might take the OK to come back, and then add on some margin to the longest delay seen.

It would be much better to actually look for the OK response rather than use a delay.  Two reasons: you might very likely get an ERROR or other response, instead of the OK, if the message could not be sent for some reason.  Secondly, looking for the OK response saves the guesswork over how long to safely make the delay.  It will allow the process, on average, to run much faster.