The triangles ABO and CBO are congruent.
Which side is equal to BC?
AAABBBCCCDDDOOO
Let's consider the corresponding congruent triangles letters:
CBO=ABO CBO=ABO CBO=ABO
That is, from this we can determine:
CB=AB CB=AB CB=AB
BO=BO BO=BO BO=BO
CO=AO CO=AO CO=AO
Side AB