$解:(1)將點(diǎn)A(3,1),B(-1,n )代入y=\frac{m}{x},$
$得\begin{cases}{ 1=\frac {m}{3}, }\ \\ {n=\frac {m}{-1},\ } \end{cases} 解得\begin{cases}{ m=3, }\ \\ {n=-3,\ } \end{cases}\ $
$∴點(diǎn)B(-1,-3),反比例函數(shù)的表達(dá)式為y=\frac{3}{x}.$
$將點(diǎn)A(3,1),B(-1,-3)代入y=kx+b.$
$得\begin{cases}{ 1=3k+b, }\ \\ { -3=-k+b, } \end{cases} 解得\begin{cases}{ k=1, }\ \\ { b=-2, } \end{cases}\ $
$∴一次函數(shù)的表達(dá)式為y=x-2.\ $
$(2)存在.將x=0代入y=x-2,得y=-2,$
$∴C(0,-2),∴OC=2.$
$若四邊形OCNM是平行四邊形,則MN//OC,$
$且MN=OC=2,設(shè)M(t,\frac{3}{t}),N(t,t-2),$
$則M=y_{m} -y_{n} =\frac{3}{t}-(t-2)=2,$
$解得t=±\sqrt {3} .\ $
$∴M(\sqrt {3} ,\sqrt {3} )或(-\sqrt {3} , -\sqrt{3}).$