Concave
Hollow
オウ
くぼ, へこ, ぼこ
凹 （オウ、くぼ, へこ, ぼこ） Concave, Hollow
凸 （トツ、でこ） Convex, Uneven
Ohh おう was the only thing he could say because of the dent 凹 in his head.