By missing, do you mean you did not designate someone for your academic LoR? Or that your LoR writer still hasn't submitted it?
If it's the former, I have never heard of anyone getting interview invites or acceptances without all the requisite letters, so I have no idea.
If it's the latter, then yes, OptomCAS will process and mail out your application without the last LoR. They will send it to the schools at a later date as soon as they get it. And yes, schools will invite you for interviews even before they get all the letters.
I know some people who are still finishing up their application right now. I don't doubt there are people who will not submit them until May. Whether they are competitive or not is another story...🙄