It is safe to say that, if they invite you to an interview, they are interested in the possibility of admitting you. At most places, the interview is just to see that you can communicate and that you know what optometry is all about.
At my school, you are pretty much guaranteed an interview you if you have so many points (they do some calculation with your GPA and OAT score). As long as you don't botch the interview, you're probably in. Usually, by the time they interview all these people, there's a full class.
I can't speak for the schools you're interested in, but an interview is always a good sign. Good luck!
If you get an interview it means you have reached the next step. Two people who are granted interviews - one with a 3.0 GPA and one with a 4.0, WILL NOT be on level playing fields, just because they both got interviews. Although some interviews are "blind", all things will be taken into consideration when the admissions committee makes their decisions. Good luck!