OPEC and other oil producers agreed on a way to monitor their compliance with last month’s historic supply deal, putting global markets on track to re-balance after more than two years of oversupply. The countries have already cut oil supply by 1.5 million barrels a day, more than 80 percent of their collective target, since the deal took effect on Jan. 1, Saudi Arabia’s Minister of Energy and Industry Khalid Al-Falih told reporters in Vienna.