B

$\because \triangle OMN$与$\triangle OAB$是以点$O$为位似中心的位似图形，$B\left(3,0\right)$，$N\left(9,0\right)$，
$\therefore \triangle OMN$∽$\triangle OAB$，相似比为$1:3$，
$\because A\left(2,1\right)$，
$\therefore$点$M$的坐标为$\left(6,3\right)$，
故选：$B$.