"B O" is a player with no rating. After each match, the players are given marks. Based on the points scored, ratings are compiled. If the player has not shown himself in any way, the score is not given.
If we are talking about "BO" in the post-match review in a newspaper or magazine, then most likely it stands for "no evaluation". That is, analysts gave all the players certain points depending on the quality and usefulness of their game, and someone remained unappreciated, for example, because they entered the field in the last minutes of the match and did not have time to show themselves either negatively or positively.