The given points are $O(0,0,0)$ and $A(1,2,-3)$
$\Rightarrow\:$ Direction ratio $(d.r.)$ of $OA\: is\:\:(x_2-x_1),(y_2-y_1),(z_2-z_1)$
Since $A$ is foot of perpendicular from origin on the plane,
$OA$ is also $\perp$ to the plane.
We know that the equation of the plane passing through $(x_1,y_1,z_1)$ is
Where $(a,b,c)$ are the direction ratio of normal $(\overrightarrow n)$=$(1,2,-3)$ and the point $A$ is $(1,2,-3)$.
$\therefore$ The equation of the required plane is $1(x-1)+2(y-2)-3(z+3)=0$
Hence equation of required plane is $x+2y-3z-14=0$