Oil prices tumbled again yesterday, eroding last week's gains, as OPEC called for co-operation from oil producing nations outside the cartel. Brent crude fell 6.3 per cent to 30.15 dollars a barrel following a 10 per cent rise on Friday, while US oil shed 7.1 per cent to 29.90 dollars. The slide came as the head of Opec Abdullah al-Badri called for all oil producing nations to work together. Al-Badri said both OPEC and non-OPEC oil producers needed to tackle oversupply to help prices rise.