"O" should be placed in the center.
Let's number the positions as:
1 | 2 | 3
---------
4 | 5 | 6
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7 | 8 | 9
It is given that "X" is placed in one of the corner position. Let's assume that its at position 1.
Now, let's take each position one by one.
• If "O" is placed in position 2, "X" can always win by choosing position 4, 5 or 7.
• If "O" is placed in position 3, "X" can always win by choosing position 4, 7 or 9.
• If "O" is placed in position 4, "X" can always win by choosing position 2, 3 or 5.
• If "O" is placed in position 6, "X" can always win by choosing position 3, 5 or 7.
• If "O" is placed in position 7, "X" can always win by choosing position 2, 3 or 9.
• If "O" is placed in position 8, "X" can always win by choosing position 3, 5 or 7.
• If "O" is placed in position 9, "X" can always win by choosing position 3, or 7.
If "O" is placed in position 5 i.e. center position, "X" can't win unless "O" does something foolish ;))
Hence, "O" should be placed in the center.
