If I were in that position, and the landlord did ask me about it, I would just let him/her know that it is legal in Wisconsin. That it is no threat to anyone, and that SOON CCW will be the law of our land. At that point if he/she still does have an issue with OC, I would ask what he/she will do when CCW does pass? Will he/she try to ban ALL carry of guns in the building? And what will happen if the criminals find out that you have a gun free apartment building? We don't have any 1000 foot gun free zone for apartments, so crime could happen.
I would not stop my OC. I would wait for the landlord to talk to me. You are doing nothing wrong. I hope your landlord sees it that way also.