Solution :

$$\eqalign{
& {\text{Equation of PO :}}\,\,\frac{{x - 1}}{1} = \frac{{y + 5}}{1} = \frac{{z - 9}}{1} = \lambda \cr
& \Rightarrow x = \lambda + 1\,;\,\,y = \lambda - 5\,;\,\,z = \lambda + 9 \cr} $$
Putting these in equation of plane :-
$$\eqalign{
& \lambda + 1 - \lambda + 5 + \lambda + 9 = 5 \cr
& \Rightarrow \lambda = - 10 \cr
& \Rightarrow {\text{O is }}\left( { - 9,\, - 15,\, - 1} \right) \cr
& \Rightarrow {\text{distance OP }} = 10\sqrt 3 \cr} $$