Oliver didn't have a chance to reply before Asch went to the Plane. And it was only Asch who left--in fact, Oliver saw him disappear. But something did happen to Oliver as well. Most likely Asch's attempt to pull him to the Plane had set it in motion, though he didn't know that for sure, but suddenly he wasn't just seeing the clearing anymore. Like the last times he had moved between worlds, he was seeing double now--both the outcropping and some sort of town.

"Asch?" Oliver called. He had been through this on several worlds already, so he knew he was going to be leaving in minutes no matter what. And while it might have been easiest to do so without having to say anything more to Asch, he couldn't help wanting to at least say good-bye to his friend. He may have still been upset about the killing, but...he found that the time they'd spent together and Asch's explanation had at least taken care of his anger. He didn't want to just disappear without a word.