Welcome to the board..
You probably actually want AND instead of or..
And does it have to be letters? or basically ANYTHING other than A or V?
What about Blanks?
Try this for starters
=AND(A1<>"A",A1<>"V",A1<>"")
This is TRUE if A1 = ANYTHING other than A or V or Blank.
Hope that helps.
OK, so to make it work for blanks as well
=AND(A1<>"A",A1<>"V")
This will make the formatting apply if ANYTHING other than "A" or "V" is entered in A1.
Actually, it will also accept "a" or "v" ...it's not case sensitive.
If you need it to be case sensitive
=AND(NOT(EXACT(A1,"A")),NOT(EXACT(A1,"V")))