How to check if a string contains one of the patterns using And & Or in If statement

Hi there,

I want to check a cell. If it doesn't meet one of the patterns below, it will return error msgbox.

1. Start with "OP" and the length is 8, or
2. Start with "DF" and the length is 10

I wrote a code like this:

    If Not (Left(cell.value, 2)) = "OP" And Len(cell.value) = 8) Or _
            ((Left(cell.value, 2)) = "DF" And Len(cell.value) = 10) then msgbox "Error"
            End if

However, it doesn't work. Could you please point out why?

Thank you so much,
Harry