No, it's easy.

"S" is one time unit on, one time unit off, one time unit on, one time unit
off, one time unit on. Then at least three time units off.

"O" is three time units on, one time unit off, three time units on, one time
unit off, three time units on. Then at least three time units off.



That assumes the sender sends correctly. If he spaces his dahs
too far apart, or runs his dits too close, decoders might get
confused. But only lids would do that.....