Number of games ending on the seventh move
Another little complexity:
there are again 8 lines of three squares, but this time it does matter in which order the four Xs were placed, as the fourth must be on the line, while the three Os could have gone into three of the other five squares in any order (providing that the Os are not three in a row). Ignoring the bracketed phrase, this gives us 8*3*6*3!*5*4*3 = 51840 possibilities. To take account of the bracket, we need to exclude cases where there are three Os in a row and three Xs in a row: none of them can be a diagonal, and if a particular row is taken with Xs, there are only two other possible rows of which one has an X, so we need to exclude 6*3*6*3!*3! = 3888 cases. So we are looking at 51840-3888 = 47952 possibilities for games ending in a win on the seventh move.