1.O9 M8 is the move.

Note that White has two key groups which are safely connected to the left edge: a four-stone group in the upper middle (at G11, H11, I11 and J11) and a three-stone group near the lower right (at K3, K4 and K5). In this position White is threatening to connect the right edge to either of these groups. Only the solid 1.O9 M8 blocks off both groups and gives Black the victory. (1.O9 N11 also looks forceful but it falls to 2.L10 M12.)

In response to 1.O9 M8 White might try 2.K7 N7, looking to follow up with 4.L10 K8. However Black can temporarily abandon O6 and play 3.L6 K8!