Answer:

OOX

Answer	Crossword Clue
OOX	Tic-tac-toe block
OOX	Nonwinning game line
OOX	Tic-tac-toe loser

OOX	Losing line
OOX	Sign of a blocked victory
OOX	"Hollywood Squares" loser
OOX	Tic-tac-toe line
OOX	Losing tic-tac-toe line
OOX	Tic-tac-toe nonwinner
OOX	Losing game line
OOX	Love letter signoff, maybe

OOX	Losing tic-tac-toe row
OOX	A tic-tac-toe loser
OOX	Near miss in tic-tac-toe
OOX	Noughts-and-crosses loser
OOX	Tic-tac-toe failure
OOX	Losing line in a children's game
OOX	Losing line in tic-tac-toe
OOX	It's for naught in noughts-and-crosses