Billy Crystal commemorated his good friend and "When Harry Met Sally" director Rob Reiner at the Oscars in a tearjerking tribute to the late Oscar nominee and his wife Michele.

After going through all of Rob's beloved films, Billy said, "To the millions who have enjoyed his films for all these years, I want you to know how many times Rob told me that it meant everything to him that his work meant something to you."

Billy was later joined on stage by some of Rob's most notable collaborators ... Christopher Guest, Jerry O'Connell, Michael McKean, Fred Savage and more linked hands as they honored their great friend.

As you know ... Rob and his wife Michele were found stabbed to death at their Los Angeles home in December. Their son, Nick Reiner, was arrested in connection with the brutal killings and charged with two counts of first-degree murder with a special circumstance of multiple murders.

Billy was among the first people called to the Reiner's home after Rob and Michele's bodies were discovered.