椭圆\(C\):\( \dfrac {x^{2}}{a^{2}}+ \dfrac {y^{2}}{b^{2}}=1\),\((a > b > 0)\)的离心率\( \dfrac { \sqrt {2}}{2}\),点\((2, \sqrt {2})\)在\(C\)上.
\((1)\)求椭圆\(C\)的方程;
\((2)\)直线\(l\)不过原点\(O\)且不平行于坐标轴,\(l\)与\(C\)有两个交点\(A\),\(B\),线段\(AB\)的中点为\(M.\)证明:直线\(OM\)的斜率与\(l\)的斜率的乘积为定值.