That's How You Prevent A Fork

$\text {CASE 1}$ :-
If $O$ plays $1$ . Then $X$ who plays optimally would play $5$ . Then $O$ would definitely lose because , in the next turn it can play either $2$ or the square to the right of the bottom $X$ . But, in the next turn, $X$ would definitely win.

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$\text {CASE 2}$ :-
If $O$ plays $2$ . Then $X$ who plays optimally would play in the square right of the bottom $X$ . Then $O$ would definitely lose because , in the next turn it can play either $3$ or $5$ . But, in the next turn, $X$ would definitely win.

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$\text {CASE 3}$ :-
If $O$ plays $3$ . Then $X$ who plays optimally would play $5$ . Then $O$ would definitely lose because , in the next turn it can play either $2$ or the square to the right of the bottom $X$ . But, in the next turn, $X$ would definitely win.

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$\text {CASE 4}$ :-
If $O$ plays $4$ . Then $X$ who plays optimally would play in the square left of the bottom $X$ . Then $O$ would definitely lose because , in the next turn it can play either $3$ or $5$ . But, in the next turn, $X$ would definitely win.

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$\text {CASE 5}$ :-
If $O$ plays $5$ . Now, the optimal player $X$ can't do anything to defeat the optimal player $O$ because now $X$ would not succeed in forming a two path trap (like the ones in the above cases).

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So, the best move for $O$ is $\boxed{5}$ .

Always look for your opponent's intent. Which move of your opponent gives them the best shot at winning? Lets give our squares coordinates ey?

(1 , 1) : (2 , 1) : (3 , 1)

(1 , 2) : (2 , 2) : (3 , 2)

(1 , 3) : (2 , 3) : (3 , 3)

Coordinates of your opponent's 'X' are (1 , 1) and (2 , 3) To give the the best opportunity to win, they need to play (2 , 1) or (1 , 3) common squares for their 3 in a row or column. Since (2 , 1) is taken, they'll try (2 , 3) next. That is why you need to play that move

If x is put in the positions 2 &5 you can see there'll be two way path for x.Again if o is put in 2 still there's a two way path.So you have to put o in the position of 5.