In addition to funds, he urged for aid groups to be guaranteed "safe and unimpeded access" to deliver humanitarian aid.
"We stand at a critical point in history", O'Brien told the Security Council on Friday.
More than 1 million children under the age of five in South Sudan are thought to be acutely malnourished, with 270,000 at risk of dying of starvation imminently if aid can not be brought to them, he said.
The world is facing the largest humanitarian crisis since 1945, the end of the World War II, the UN's humanitarian chief said, adding that without a collective effort, people in conflict-stricken countries will "starve to death" or "die from diseases".
United Nations and food organisations define starvation as when more than 30% of children under the age of five suffer from acute malnutrition and mortality rates are two or more deaths per 10,000 people every day, among other criteria.
"The starvation in South Sudan is man-made", he said.
Following that immediate impact will be the repercussions of hungry children being unable to attend school, economic and social development processes slowing and reversing, instability due to displacement and "livelihoods, futures and hope lost", he said.
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He announced that Secretary-General Antonio Guterres will chair a pledging conference for Yemen on April 25 in Geneva.
More than 7.5 million people in the country need aid, he said, over one million more than past year. "Parties to the conflict are parties to the starvation - as are those not intervening to make the violence stop", O'Brien said, asking local authorities to take their promises of unconditional help on paper into "action on the ground". Estimates in December showed that there were 75,000 children at risk of starving to death. The situation in Ganyiel is dire, with thousands of people having fled to the area from famine-stricken Leer and Mayendit counties.
"That is three million people more than in January", he added. "With everything lost, women, boys, girls and men now move to urban centres".
On-going conflicts and civil insecurity are further intensifying the food insecurity of millions of people across the region, and there is already widespread displacement and other cross-border spill over. We need access now.
The humanitarian chief said in Somalia the current indicators mirrored the tragic picture of 2011 in the country when some 260,000 people died of starvation.
The conflict, which has left around 20,000 people dead and forced more than 2.6 million others to flee their homes, has aggravated an already hard humanitarian situation in one of the poorest regions of the world. "Yemen, northern Nigeria and Somalia are also on the brink of famine", analysts said.