Hellos. First of all, when she says "Okay." It probably is because she's shocked and she doesn't know what to say. Imagine the awkwardness when a guy(hopefully a nice and sweet guy) comes over to her and suddenly tells her that he likes her and it's not just a simply crush, and you have a boyfriend. You would be just standing there and all you can manage is okay. Of course, to you it's not going to be a satisfactory answer but you probably can't expect her to say anything else.
No matter whether she doesn't have any feelings for you, it will make her think about it, even if she doesn't plan to break her relationship or anything.
There's definitely a chance that she might go out with you in the end. You know relationships are not going to last forever and maybe they will break up soon. Although it may make things awkward between you and her, it's good that you've said it. You are open about your feelings towards her and...that's well, good. You never know what will happen.
It's good that you have supporting friends. Right now I think there's nothing you can do but like your friends say, stay optimistic.
Also, know that if it can't work out between you and her, life is still young. There are plenty of chances to meet other girls, so don't get too upset.
Good luck!