AngularFire Tic Tac Toe Game
I am not an angular expert, but this 

var piece;
if(returnedData.oIsAvailable == true) {
    piece = 'x';
}
else {
    piece = 'o';
}
$scope.myPiece = {
    val:piece
};


could be 

$scope.myPiece = {
    val: returnedData.oIsAvailable ? 'x' : 'o'
};


Also


I know the firebase API calls the object returned by new Firebase a (root) reference. It still irks me since my brain keeps thinking what is ref?? I would probably go against the grain and call it db, but that could be because I am simply not familiar enough with the API
if(checkThisGame.oIsAvailable == true){ should be if(checkThisGame.oIsAvailable){ or if(checkThisGame.oIsAvailable === true){
the game is well documented
JsHint.com finds a number of small things, you should use the site