Antonio was the son of Captain-General Miguel de Oquendo, who died in October 1588 when his ship foundered off Pasajes, while coming back from the ill-fated campaign of the Armada Invencible.

In 1619 he temporarily replaced Juan Fajardo de Tenza, 1st Marquess of Espinardo, arrested for insubordination, as commander of the Squadron of the Ocean, the Atlantic high seas navy.

Nevertheless, he was barred from command of the treasure fleet for four years and condemned to pay an indemnity of 12,000 ducats for having caused the loss of the galleons Espíritu Santo and Santísima Trinidad near Cuba through neglect of duty.

On 15 September he was intercepted near the Strait of Dover by the squadron of Dutch Lieutenant-Admiral Maarten Tromp, who was reinforced two days later by a flotilla of Vice-Admiral Witte de With.

Though the Dutch force was rather small, consisting of only seventeen vessels, it managed by a clever use of the line-of-battle to severely damage the larger and crowded Spanish ships.

De Oquendo feared that if he entered the narrow channel to Dunkirk, he would be trapped in that port, so he opted to take refuge in The Downs, in neutral English waters at the coast of Kent.

As Charles I of England had concluded a secret treaty with Spain against the Dutch, De Oquendo hoped to move his troops to Flanders by means of English shipping.