Here's a good website to check out about this. Sometimes kitties get very finicky about exactly how their litterbox is set up, and where. You might want to (at least for now) set up the litterbox where Osiris is going, if it's in a certain spot.
My boy, Hobbes, did the same thing, and it turned out he wanted one litterbox to poop in and one to pee in, he didn't like hooded litterboxes, and he liked it pristinely clean. I found a couple websites that helped me understand exactly what it was that I could try that would help, and it really did. One good thing is the fact that he's now outgrown his pickiness about wanting two and wanting them in such perfect condition, now that he's about a year old! Hopefully Osiris will do the same!
Here are a couple websites that should help:http://www.larimerhumane.org/services/litterboxinfo.cfmhttp://www.thepetprofessor.com/secAr...s_and_cats.asphttp://www.heartlandhumane.org/pub/petcare_litterbox
A couple tips:
Cats don't usually like the plastic liners some people use in litterboxes.
Some cats don't like the hooded boxes.
Cats at times don't like the box to be anywhere that they also have to eat and drink.
Sometimes the problem is just that the box isn't somewhere private enough. They only like to "go" somewhere that is low-traffic, quiet, and very private.
Hope all that helps!