X and O

The game of Tic-tac-toe is played in a large square divided into nine small squares.
[1] Each of two players in turn places his or her mark -usually X or O-in a small square.

---------- X O __
------X __ __
------O O --X

[2] The player who first gets three marks in a horizontal, vertical, or diagonal line wins.
[3] A player will always place his or her mark in a line that already contains (a) two of his or her own marks or (b) two of his or her opponent's marks- giving (a) priority over (b).
Only the last mark to be placed in the game shown above is not given.
Which mark-X or O- wins the game?
{Level-Medium/Hard}