In 1629 he concluded the advantageous Truce of Altmark with Poland-Lithuania. Prior to this, in September 1628, he arranged a joint occupation of Stralsund with Denmark to prevent that important fortress from falling into the hands of the Imperialists.

Oxenstierna was not only highly successful within the diplomacy. During these years, he was entrusted with various important assignments in which he succeeded, such as gathering money and troops for the attack