Too easy
This was fantastic except it was too easy... Here are 3 tips: 1. Make it so if the difficulty is turned all the way up, it's harder than now. 2. Make it so the person who wins doesn't keep going first (The point to that is why it's so easy almost. See when I go first, I'll always go in a corner right? Well the difficulty when it's at its highest won't go in the places that can stop my strategies, but when the computers doing it, I can mess up its... [Whenever I do a corner first there's always a way I can win... I play with my brother, me and him NEVER have beaten each other.]) 3. Fix the difficulty slider, it's a little glitchy. Well other than that, excellent job ^_^, you did great with the functionality, other than what I exposed, the backgrounds and music set-ups were nice. Keep it up!
(Tic-Tac-Toe is a flawed game anyway, and see I'm just saying to get it so they can stop you from doing the corner thing SOMETIMES and not NEVER, if you do it right that is...) ^_^