The British charity said people were falling ill from eating grass and weeds. A four-year civil war, poor harvests and rising food prices have been blamed for the widespread hunger in the world's youngest nation.
Oxfam on Tuesday urged urgent action to prevent millions of people being hit by famine in South Sudan.
The British charity's call for action comes just days after the latest rounds of peace talks in Addis Ababa failed to bring an agreement to end a four-year conflict in the country.
Last week, a report said a fifth of households in the town of Pibor in eastern Boma state could already be classified as suffering from famine levels of hunger.
Pibor has also seen food availability drop sharply after pests and flooding destroyed harvests.
As the rainy season sets in, much of the food supplies would be required to be flown in from capital Juba, making it unaffordable for many.
Eating grass and weeds for survival
Nicolo Di Marzo, Oxfam acting Country Director in South Sudan said: "Waiting for a formal declaration of famine is not good enough. People are already starving and desperate. A woman in Pibor said that her family has resorted to cooking grasses and weeds that are making them sick -- but that they have no choice if they want to survive."
"This is not an isolated problem. Urgent action is needed now to save lives in Pibor and across South Sudan," he added.