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The one with the winning strategy is quite surprisingly

X

First, let us note that

if X starts anywhere else than the center, O wins on their first move like this

 |X *|     |*X |
 
 |   |     |O* |
 
 |* O|     |   |

First options

When X starts in the center, O has two options, corner, which gives X zero winning moves or middle, which gives X six winning moves

 |O  |     |*O*|

 | X |     |*X*|

 |   |     |* *|

Case 1

If O goes for the corner, X wins like this

 |O  |    |O X|    |OXX|

 | X | -> | X | -> |*X |
 
 |   |    |O  |    |O* |

Case 2

So O should go for the middle. Then, if X goes anywhere else than one of the more distant corners, O wins immediately like this

 | O |     |XO |     |*O |
 
 |OX |     |*X |     |XXO|

 |*X*|     |* O|     |*  |

Case 2.1

So X will go for one of the bottom corners. Then the game will end like this

 | O |    | OO|    |*OO|

 | X | -> | X | -> | X |
 
 |   |    |X  |    |XX*|

The one with the winning strategy is quite surprisingly

X

First, let us note that

if X starts anywhere else than the center, O wins on their first move like this

 |X   *|     |* X  |
 |     |     |O *  |
 |*   O|     |     |

First options

When X starts in the center, O has two options, corner, which gives X zero winning moves or middle, which gives X six winning moves

 |O    |     |* O *|
 |  X  |     |* X *|
 |     |     |*   *|

Case 1

If O goes for the corner, X wins like this

 |O    |    |O   X|    |O X X|
 |  X  | -> |  X  | -> |* X  |
 |     |    |O    |    |O *  |

Case 2

So O should go for the middle. Then, if X goes anywhere else than one of the more distant corners, O wins immediately like this

 |  O  |     |X O  |     |* O  |
 |O X  |     |* X  |     |X X O|
 |* X *|     |*   O|     |*    |

Case 2.1

So X will go for one of the bottom corners. Then the game will end like this

 |  O  |    |  O O|    |* O O|
 |  X  | -> |  X  | -> |  X  |
 |     |    |X    |    |X X *|

The one with the winning strategy is quite surprisingly

X

and the strategy is

Start in the center. First, let us note that if X starts anywhere else, O wins on their first move like this

 |X *|     |*X |

 |   |     |O* |

 |* O|     |   |

First options

When X starts in the center, O has two options, corner, which gives X zero winning moves or middle, which gives X six winning moves

 |O  |     |*O*|

 | X |     |*X*|

 |   |     |* *|

Case 1

If O goes for the corner, X wins like this

 |O  |    |O X|    |OXX|

 | X | -> | X | -> |*X |

 |   |    |O  |    |O* |

Case 2

If O goes for the middle, X wins like this

 | O |    | OO|    |XOO|

 | X | -> | X | -> | X |

 |   |    |X  |    |X  |

Case 2 continued

As we can see, X has now two possible winning lines but it is O's turn. If O tries to block one of the winning lines, X has exactly one winning move. However, if O doesn't block either of the lines, then X will eventually win with any of the three remaining cells so X has three winning moves. Thus, either way, X will eventually win.

The one with the winning strategy is quite surprisingly

X

First, let us note that

if X starts anywhere else than the center, O wins on their first move like this

 |X *|     |*X |

 |   |     |O* |

 |* O|     |   |

First options

When X starts in the center, O has two options, corner, which gives X zero winning moves or middle, which gives X six winning moves

 |O  |     |*O*|

 | X |     |*X*|

 |   |     |* *|

Case 1

If O goes for the corner, X wins like this

 |O  |    |O X|    |OXX|

 | X | -> | X | -> |*X |

 |   |    |O  |    |O* |

Case 2

So O should go for the middle. Then, if X goes anywhere else than one of the more distant corners, O wins immediately like this

 | O |     |XO |     |*O |

 |OX |     |*X |     |XXO|

 |*X*|     |* O|     |*  |

Case 2.1

So X will go for one of the bottom corners. Then the game will end like this

 | O |    | OO|    |*OO|

 | X | -> | X | -> | X |

 |   |    |X  |    |XX*|

The one with the winning strategy is quite surprisingly

X

and the strategy is

Start in the center. First, let us note that if X starts anywhere else, O wins on their first move like this

 |X *|     |*X |

 |   |     |O* |

 |* O|     |   |

First options

When X starts in the center, O has two options, corner, which gives X zero winning moves or middle, which gives X six winning moves

 |O  |     |*O*|

 | X |     |*X*|

 |   |     |* *|

Case 1

If O goes for the corner, X wins like this

 |O  |    |OX |    |OXX|    |OXX|

 | X | -> | X | -> | X | -> |*XX|

 |   |    | O |    |OO |    |OO*|

Case 2

If O goes for the middle, X wins like this

 | O |    | OO|    |XOO|

 | X | -> | X | -> | X |

 |   |    |X  |    |X  |

Case 2 continued

As we can see, X has now two possible winning lines but it is O's turn. If O tries to block one of the winning lines, X has exactly one winning move. However, if O doesn't block either of the lines, then X will eventually win with any of the three remaining cells so X has three winning moves. Thus, either way, X will eventually win.

The one with the winning strategy is quite surprisingly

X

and the strategy is

Start in the center. First, let us note that if X starts anywhere else, O wins on their first move like this

 |X *|     |*X |

 |   |     |O* |

 |* O|     |   |

First options

When X starts in the center, O has two options, corner, which gives X zero winning moves or middle, which gives X six winning moves

 |O  |     |*O*|

 | X |     |*X*|

 |   |     |* *|

Case 1

If O goes for the corner, X wins like this

 |O  |    |O X|    |OXX|

 | X | -> | X | -> |*X |

 |   |    |O  |    |O* |

Case 2

If O goes for the middle, X wins like this

 | O |    | OO|    |XOO|

 | X | -> | X | -> | X |

 |   |    |X  |    |X  |

Case 2 continued

As we can see, X has now two possible winning lines but it is O's turn. If O tries to block one of the winning lines, X has exactly one winning move. However, if O doesn't block either of the lines, then X will eventually win with any of the three remaining cells so X has three winning moves. Thus, either way, X will eventually win.