I thought hän was implied in the word osaako. I thought there is no need to write hän.
"hän" is necessary. 1st person (minä, me) and 2nd person pronouns (sinä, te) can be left out, but 3rd person pronouns (hän, he) must always be included.
You have to include 3rd person pronouns to avoid ambiguity. "Osaat" can only mean "you know how" but "osaa" could be hän, se, Joni, Roosa, etc. knows how