oğlanı is "the boy", when it is the object of the sentence (the part being acted upon). It is displayed as a hint because in another sentence it will be correct, but not necessarily in this one. (It's not correct here because it asks for boy, not the boy.) This is a limitation of Duolingo, not the course creators' fault.
So O, oğlanı görüyor is "he sees the boy", but "the/a boy sees him" would be Oğlan onu görüyor.
I just wrote 'çocuk’ and it displayed it as a wrong answer but isn't an option given by duolingo