O'Sullivan Gambit

The O'Sullivan Gambit, 3...b5, departs from the usual 3...d6 of the Alekhine Defense by giving up the b-pawn instead of challenging the e5-pawn. The point is to draw White's light-squared bishop to b5, where it lacks protection and can become a target for checks along the fifth rank and the a5-e1 diagonal. The best-known trap runs 4. Bxb5 c5 5. dxc5?? Qa5+, when the queen checks the king and picks up the loose bishop. White does better to keep the extra pawn without grabbing on c5 in that line.

Accepting with 4. Bxb5 is both the most common reply and, in theoretical terms, the strongest; engines consider White clearly better. White can also ignore the offer with a developing move such as 4. Nf3, or with 4. c4, which hits the knight and gains space. In online games Black's practical results are good, including after 4. Bxb5, while 4. c4 gives White the best score among the main replies. The gambit remains a rare surprise rather than a mainstream line.