OMO Public VS. Bank (Selling securities): Let’s assume that the required reserve ratio is 0.2. The money multiplier is then 5.

If the Fed sells $1000 worth of securities to commercial banks, then excess reserves will decrease by $1000, so the money supply will decrease by $1000*5=$5000.
If the Fed sells $1000 of securities to the public, then after the transaction is cleared, the bank will have $1000 less in securities. $200 of that money can be taken from Federal reserves, and so excess reserves only decrease by $800, causing the money supply to decrease by $800*5=$4000