Chess Openings Lab
a8b7c6d5e4f3g2h1

Blackmar-Diemer Gambit Declined: O'Kelly Defense

The O'Kelly Defense is 4...c6, a quiet move that declines to take on f3 and supports a later ...e5 or ...Qb6. White can take back on e4 at once, while Black aims to open the center with ...e5 and reach a simple endgame.

Moves
1.d4 d5 2.e4 dxe4 3.Nc3 Nf6 4.f3 c6
ECO
D00
Engine
−0.26 · level (Stockfish 17.1, depth 20)
Played
367 games in our lichess sample
White scores
56.8%

Key ideas for both sides

White

  • 5.fxe4 is the engine's first choice; after 5...e5 6.dxe5 Qxd1+ 7.Nxd1 the knight recaptures and White stays solid.
  • 5.Nxe4 is a cleaner option: trade with Nxf6+ and play c3 and h4, as in the second engine line.
  • With the e-pawn back on e4, White has a classical center and no material deficit.
  • Avoid slow moves like 5.a3, which allow ...exf3 and a pin with ...Bg4.

Black

  • After 5.fxe4, hit back with 5...e5. The queen trade and ...Nxe4 regain the pawn and free Black's game.
  • ...Qb6 attacks b2 and d4 whenever White's pieces leave them loose, a common resource here.
  • Against 5.Nxe4, recapture on f6 with the other knight and develop with ...Bf5 and ...e6, a Caro-Kann setup.

Plans and pawn structure

If White plays fxe4 and Black replies ...e5, the queens come off and the game is a level endgame. Black's knight lands on e4, the bishop goes to e6, and both sides fight over the open d-file. After 5.Nxe4, White keeps the queens and builds with c3 and Nf3, while Black uses ...Nbd7, ...Bf5 and ...e6 for a Caro-Kann type position. In both cases the gambit has been returned, so White plays for a slight structural edge.

What the engine plays from here

Stockfish 17.1 at depth 20 rates the position −0.26: level. Its three top choices for White:

  1. −0.26 5.fxe4 e5 6.dxe5 Qxd1+ 7.Nxd1 Nxe4
  2. −0.34 5.Nxe4 Nbd7 6.Nxf6+ Nxf6 7.c3 Bf5
  3. −0.67 5.a3 exf3 6.Nxf3 Bg4 7.h3 Bxf3

Pawns, from White's side.

How it scores on lichess

367 rated lichess games reached this position; White scored 56.8%.

under 120050.0%
1200–159964.2%
1600–199959.0%
2000 and up51.4%

Light: White wins. Grey: draws. Dark: Black wins. About the sample.

How to play against it

Take on e4 right away; either 5.fxe4 or 5.Nxe4 gets your pawn back. If you choose fxe4, be ready for the queen trade after ...e5 and recapture with the knight. Avoid 5.a3 or other waiting moves, since then Black takes on f3 and pins with ...Bg4 while you are still a pawn down.

Questions

What are the moves of the Blackmar-Diemer Gambit Declined: O'Kelly Defense?

The Blackmar-Diemer Gambit Declined: O'Kelly Defense starts 1.d4 d5 2.e4 dxe4 3.Nc3 Nf6 4.f3 c6. Its ECO code is D00.

Is the Blackmar-Diemer Gambit Declined: O'Kelly Defense good for Black?

Stockfish 17.1 rates the position −0.26 at depth 20, which is level. In 367 lichess games from our sample, White scored 56.8%. Good for Black players who want to avoid gambit theory and accept a dry, level endgame.

What is the main idea of the Blackmar-Diemer Gambit Declined: O'Kelly Defense?

Black prepares ...e5 and ...Qb6 rather than grabbing a second pawn on f3. White usually takes back on e4, and the game becomes a balanced fight with Black aiming for ...e5 and an early queen trade.

What should White play next?

The engine's first choice is fxe4, with Nxe4 and a3 close behind. 5.fxe4 regains the pawn at once, and after ...e5 White accepts a queen trade with a sound position.

Updated October 2026. Sources and method.