Yeah, 5 years in an attic has dried the oak and may have caused the glue to fail in that joint. I had two 50 year old Heywood Wakefield table tops split from attic storage and they could not be pulled back together. Oddly enough, the other two joints in your post haven't failed. If you can't force the joint closed with screw clamps, I'm not sure bolts and nuts will either. I don't think hydrating the open joint with wet water will move the wood back together. I assume the picture is the top of the post, so any fix will be visible in the finished piece. Can you turn the post over to use it? If it were mine, I'd use my 3//4" pipe clamps placed one against the other the length of the opening to see if it will close and if so, I'd try thinned epoxy in the joint and all of those clamps, but the wood may resist and crack next to the joint. The suggestion to resaw, plane, and reglue sounds good, but I'd be thinking of replacing the damaged post with a new one. More costly, but a lot less time consuming and less troublesome.