Andre: At the end of the sequence, taking away the last liberties of the black l-shape, it classifies P1 in stead off O1 as incorrect.
However i think that any sequence winning the liberty race is good.
Neri: O1 is better because R2 ko treat is smaller in this case.
I think there is a more simple combination that works and is claimed "incorrect" : Q19 R19 S18 S19 R18 P19 T19 S19 N19