OK, so if you are playing another human with reasonable intelligence it us unlikely that you will be able to beat them. Computers on the other hand are much easier to beat. Not because their programmers aren’t intelligent, but because their programmers have to guess what the human opponent will do. So, if you do something that the programmer didn’t anticipate, then you can win.
First MoveDo not, I repeat, DO NOT start in the middle square! This is a strategic dead end. It leaves your opponent only one reasonable move, taking a corner. If instead, you take a corner first, your opponent now has several viable moves. The more moves available, the more room for mistakes.
A computer opponent will now, almost always, take the center square.
Next MovesYour response is then to take the corner opposite your first move so that you have two Xs separated by an O.
Nearly all computer programs will not understand your move and default to taking what it thinks is the best move, which is one of the remaining corners. If this happens, you’ve won the game.
If, however, O takes the top middle square, the programmer anticipated this strategy and you’re doomed to a tie.
The SetupYou see that the computer has two Os in a row. You have to put your X in the lower left to block, but in doing so you set up two sets of Xs with two in a row. The computer will move to block one of those rows, leaving you with the winning move.
You place your X to make three in a row and you’ve won.
You can try this out for yourself. There are a couple variations of this strategy that may or may not work depending on the programmer. The screenshots above are taken from a tic-tac-toe program I wrote. It is beatable at all levels. You can download it by clicking on the link. (Windows only) Enjoy.