probably something along the lines of:
Private Sub OptButtonHP_Change()
With Sheets("Lists")
If OptButtonHP Then
cmbboxModel.List = .Range("A2", .Cells(.Rows.Count, "A").End(xlUp)).Value
Else
cmbboxModel.List = .Range("B2", .Cells(.Rows.Count, "B").End(xlUp)).Value
End If
End With
End Sub
or:
Private Sub OptButtonHP_Change()
With Sheets("Lists")
If OptButtonHP Then
cmbboxModel.RowSource = .Range("A2", .Cells(.Rows.Count, "A").End(xlUp)).Address(external:=True)
Else
cmbboxModel.RowSource = .Range("B2", .Cells(.Rows.Count, "B").End(xlUp)).Address(external:=True)
End If
End With
End Sub