Equation of line passing through $(1,2,1)$ and $(-2,0,3)$ is given by
$$
\begin{aligned}
& \frac{x-1}{-2-1}=\frac{y-2}{0-2}=\frac{z-1}{3-1}=\lambda(\text { say }) \\
\Rightarrow & \frac{x-1}{-3}=\frac{y-2}{-2}=\frac{z-1}{2}=\lambda
\end{aligned}
$$
Any point on this line has coordinate $A(-3 \lambda+1,-2 \lambda+22 \lambda+1)$.
Equation of plane passing through the points $(0,0,0),(0,0,4)$ and $(2,1,0)$ is given by
$$
\begin{array}{cccc}
\left|\begin{array}{rrr}
x-0 & y-0 & z-0 \\
0-0 & 0-0 & 4-0 \\
2-0 & 1-0 & 0-0
\end{array}\right|=0 \\
\left|\begin{array}{lll}
x & y & z \\
0 & 0 & 4 \\
2 & 1 & 0
\end{array}\right| =0 \\
\Rightarrow -4(x-2 y)=0 \Rightarrow x-2 y=0
\end{array}
$$


Since, $A$ lies on above plane
$$
\begin{aligned}
& \therefore \quad-3 \lambda+1-2(-2 \lambda+2=0 \\
& \Rightarrow \quad-3 \lambda+1+4 \lambda-4=0 \\
& \Rightarrow \quad \lambda-3=0 \Rightarrow \lambda=3 \\
& \therefore \text { Coordinates of point } A \text { are }(-3 \times 3+1,-2 \times 3+2 \\
& 2 \times 3+1), \text { i.e. }(-8,-4,7) \\
& \therefore \quad \text { OA }=-8 \hat{\mathbf{i}}-4 \hat{\mathbf{j}}+7 \hat{\mathbf{k}}
\end{aligned}
$$