My advice would be to go for the mid size if you can. I dont know what will give you the best shot in OCI, but I have friends that did in house jobs there first summer, and the places didnt do offers. So, if 3L OCI falls through, you are going to want to have something to fall back on, and the mid size firm is going to be more likely to wanna keep you around.
Firms are still giving out interviews and stuff, so dont give up yet on big law this year.