The problem lies in your use of this expression:
[tbl OAR]![Disposition])=IIf([Forms]![frm User selection report form]![Disposition_cmb]=Null,"*",[Forms]![frm User selection report form]![Disposition_cmb])
As you've seen, it works fine if the user makes a selection. But if the user leaves the combo box empty, the expression evaluates to this:
[tbl OAR]![Disposition])="*"
This expression does a literal comparison between Disposition and *. That obviously doesn't work. What you need is something like [Disposition] Like "*", but you won't get it using the expression structure you have because you have the "=" operator in there.
I will work on the problem and see if I can come up with a quick fix. Otherwise hopefully someone else can post a solution.