In blitz (lightning) chess: If my opponent knocks over a piece when moving then presses his clock before replacing the piece is this illegal? If so do I stop the clock and call the DOP?
According to FIDE Handbook:
Article 7: Irregularities
If a player displaces one or more pieces, he shall re-establish the correct position in his own time. If necessary, either the player or his opponent shall stop the chessclock and ask for the arbiter’s assistance. The arbiter may penalise the player who displaced the pieces.
That means, yes, you can stop the clock and call the arbiter for your assistance if he did not adjust the pieces in his own time.