Nice, good job! A few things:

It’d be nice if there were a way to reset it in game. I couldn’t find it, so I had to reload the page every time, which isn’t a huge deal but not having to wait for the loading time would be faster :)

Also, if the board is filled up but nobody wins, it should probably say something about it being a draw. It seems to just think it’s O’s turn even though O can’t put anything down.

One more nice touch you could add is that when a player wins, it could draw the line showing the three in a row. Just for show :)

Overall, nice job!