Highlight Cell When Already input Text with Condional Formatting

I Have make the table game like Tic Tac Toe (“O” & “X” ) in 15 rows and 15 column , the rule to win is to make 5 by rows, column, or diagonal, but i want to mark the cell with conditional formatting when “O” already turn then it will be highlighted so “X” knows the last turn of “O” , and if already get the 5 cell to win it will be highlighted too. can you help me to make the formula in conditional formatting… thank’s for the clue and guidance please…