When you see Odie go onto the porch to lay with Garfield, you see that Odie has a collar on. But after Garfield locks Odie out of the house Odie has NO collar on. And John even says in the movie that he forgot to put the collar back on Odie after the bath.
If i remember correctly , he comes out of the dog door with it OFF first ,thenwhen Garfield goes back in, Odie has it On. Just making sure you had everything correct.