$\overset{\Delta }{\mathop{OAH}}\,\cong \overset{\Delta }{\mathop{OBH}}\,\Rightarrow \overline{AH}=\overline{HB}=4$  $\overset{\Delta }{\mathop{OHB}}\,:O{{B}^{2}}=O{{H}^{2}}+H{{B}^{2}}$  ${{5}^{2}}=O{{H}^{2}}+{{4}^{2}}$  $25-16=O{{H}^{2}}$  $O{{H}^{2}}=9\Rightarrow OH=\sqrt{9}=3$