The total number of combinations of Xs, Os, and blanks is somewhere between 2048 and about 4048 (Estimating here. The total combination is 19683, but since the number of Xs and Os will always be equal or off by 1, many cases are eliminated). Thus you could represent the board in 11 or 12 bits.
You could also deduce whose turn it was based on the number of Xs and the number of Os (assuming X always go first), thus eliminating the need for a bit to keep track of the turn.