At the outbreak of hostilities, Admiral Yi had sent out his fleet out on a naval exercise. Upon hearing that Pusan had been captured, Yi immediately set out on an east course to Pusan, hoping to block Japanese naval advances along the coast to aid their land forces.

His first encounter at Okpo was a decisive victory, destroying almost half of the ships of the anchored Japanese fleet of Todo Takatora. Prior to the Okpo Campaign, Yi mainly patrolled the seas near his Jeolla Province, to fortify its position before he began moving westward, due to the call for help from Admiral Won Gyun. A day later, after destroying an additional 18 Japanese transports in nearby waters (at Happo and Jeokjinpo), Yi Sun-sin and Won Gyun parted ways and returned to their home ports after receiving news of the fall of Hanseong.

However, Yi treated each battle with extreme care and made certain that he suffered few serious casualties. From his Okpo battle, the only casualty was a minor gunshot wound on an oarsman from stray musket fire. The Battle of Okpo caused anxiety and nervousness among the Japanese, because afterward Yi began to deploy his navy to attack Japanese supply and carrier vessels.