Moving second in gomoku is hard. X starts with an extra mark and a natural lead in the race of threats, and a careless O can be lost before move fifteen. But the second player is far from helpless. With a clear plan, O can reach the middle game on equal terms — and often take over the initiative once X's first attack runs out of steam.
This article is that plan: how to answer the first moves, how to block in a way that also builds, and how to recognise the moment to strike back.
The mindset: slow X down, build while you do it
As O, you have two jobs in the opening, and you must do them with the same moves:
- Slow down X's shape, so X does not get the crossing points that produce double threats.
- Build your own lines, so that when X's attack pauses you have threats ready.
A move that only blocks wastes half its value. A move that only builds, far from X's shape, lets X develop freely. The ideal O move does both.
The first reply
After X's centre start, answer adjacent — directly beside h8 or diagonally next to it. The difference between these two families is discussed in direct and indirect openings. What matters more than which you choose is that you stay close. A reply two or three points away gives X a free move to build.
Moves three to eight: deny the crossings
In the early moves X will try to place marks where two of its potential lines cross, because those points become the double threats of the middle game. Your most valuable skill as O is seeing X's crossing points before X uses them.
A simple way to find them: look at X's marks in pairs. For each pair on different lines, where would a third mark extend both lines? Those points are the crossings. If one of them also helps your own shape, take it.
In this diagram, if X gets h7, X will have a column two and an anti-diagonal three (g8–h7–i6) at once, with plenty of room. If O takes h7 instead, both of those potential lines are cut. It is a quiet move — no immediate threat — but it removes the foundation of X's next attack.
Blocking so that it builds
Sooner or later X will make an open three, and you will have to block. Now the question is which end. The two ends of an open three are almost never equal:
- One end may sit next to your existing marks, so the block also extends your own line.
- One end may sit on another of X's potential lines, so the block also cuts a second X line.
- One end may be closer to the edge or to other O marks, leaving X less room to continue.
Ask these questions before every block. A block that also makes an O two, or cuts a second X line, is often worth a whole tempo more than the other end.
Counting tempo
Every X threat you answer is a move X spent and you spent. Neither gained anything — except that X kept the initiative. The balance shifts only when one of your answers also creates a threat that X must answer. That is the moment the initiative changes hands.
So the question to ask every time you block is simple: can this block be a threat too? If your blocking move makes an O open three, X must deal with it — and now you are the one asking questions. Read more in keeping the initiative.
When to strike back
Look for your chance when:
- X's last move did not create a threat. X is building quietly, which means you have a free move. Use it to make your own open three or to set up a crossing point.
- You have a four available that lands somewhere useful. A four forces X to answer at one exact point. If that point is harmless and your four-mark builds a second O line, you have gained a tempo.
- X's shape is cramped. If X's lines are running into your marks and the edges, X's attack is losing power, and it is time to build your own.
Many second-player wins follow the same pattern: defend accurately through the opening, then use one well-placed block that doubles as a threat, and take over.
Under blocked ends
Vietnamese caro rules help O. Every time you block one end of an X line, remember it: that line needs only one more O mark at the other end to die permanently. In the middle game, a single O move that completes the wall around an X line can be enough to neutralise X's whole attack in that direction. Keep a mental list of "half-dead" X lines and look for moves that finish them while building your own shape.
Common mistakes as O
- Playing far away to "start your own game". X simply builds a winning shape.
- Blocking the nearest end automatically. Check both ends every time.
- Ignoring quiet X moves. The crossing points are created by quiet moves, not by threats.
- Making early fours. An O four that lands its forced reply somewhere useful for X is a gift.
- Panicking. Most X attacks can be defended if you answer threats in the right order. See threat priority order.
A realistic goal for O
Beginners often measure their games as O only by wins and losses. A better measure in the first weeks is how long you keep the game balanced. If X's first open three appears at move nine and you block it well, and X's next real threat does not come until move fifteen, you are defending successfully even if you eventually lose. Each game, try to push X's decisive combination a little later.
Practice
Play a series of games as O against the computer on the home board, starting at an easy level. After each game, find the first moment you created a threat X had to answer. Try to make that moment come a little earlier each game. Replaying from your account makes this easy.
Frequently asked questions
Can the second player win at gomoku?
Yes, very often in human play. The first player has an advantage, but accurate defence and a timely counterattack win plenty of games for O.
Should O always block an open three immediately?
Almost always, unless O can make a four first that forces X to answer. Even then, the open three must be blocked afterwards.
What is the most important skill for O in the opening?
Seeing X's crossing points before X uses them, and blocking in ways that also build O's own lines.
Does the blocked-ends rule help the second player?
Yes. It lets O neutralise X's lines permanently with a second end mark, which makes defence more effective.
